[PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

 From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
From: juncheng bai <baijuncheng@unitedstack.com>
Date: Mon, 15 Jun 2015 18:34:00 +0800
Subject: [PATCH] storage:rbd: make the size of request is equal to the
  size of the object

ensures that the merged size of request can achieve the size of
the object.
when merge a bio to request or merge a request to request, the
sum of the segment number of the current request and the segment
number of the bio is not greater than the max segments of the request,
so the max size of request is 512k if the max segments of request is
BLK_MAX_SEGMENTS.

Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
---
  drivers/block/rbd.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 0a54c58..dec6045 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
  	segment_size = rbd_obj_bytes(&rbd_dev->header);
  	blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
  	blk_queue_max_segment_size(q, segment_size);
+	if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
+		blk_queue_max_segments(q, segment_size / PAGE_SIZE);
  	blk_queue_io_min(q, segment_size);
  	blk_queue_io_opt(q, segment_size);

-- 

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[Ilya Dryomov]

On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
<baijuncheng@unitedstack.com> wrote:
> From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
> From: juncheng bai <baijuncheng@unitedstack.com>
> Date: Mon, 15 Jun 2015 18:34:00 +0800
> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>  size of the object
>
> ensures that the merged size of request can achieve the size of
> the object.
> when merge a bio to request or merge a request to request, the
> sum of the segment number of the current request and the segment
> number of the bio is not greater than the max segments of the request,
> so the max size of request is 512k if the max segments of request is
> BLK_MAX_SEGMENTS.
>
> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
> ---
>  drivers/block/rbd.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 0a54c58..dec6045 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
>         segment_size = rbd_obj_bytes(&rbd_dev->header);
>         blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>         blk_queue_max_segment_size(q, segment_size);
> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>         blk_queue_io_min(q, segment_size);
>         blk_queue_io_opt(q, segment_size);

I made a similar patch on Friday, investigating blk-mq plugging issue
reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
AFAIU there is no point in setting to anything bigger since the bios
will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
256, this gives is 1M direct I/Os.

Thanks,

                Ilya

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/15 21:03, Ilya Dryomov wrote:
> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>>  From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
>> From: juncheng bai <baijuncheng@unitedstack.com>
>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>>   size of the object
>>
>> ensures that the merged size of request can achieve the size of
>> the object.
>> when merge a bio to request or merge a request to request, the
>> sum of the segment number of the current request and the segment
>> number of the bio is not greater than the max segments of the request,
>> so the max size of request is 512k if the max segments of request is
>> BLK_MAX_SEGMENTS.
>>
>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>> ---
>>   drivers/block/rbd.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>> index 0a54c58..dec6045 100644
>> --- a/drivers/block/rbd.c
>> +++ b/drivers/block/rbd.c
>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
>>          segment_size = rbd_obj_bytes(&rbd_dev->header);
>>          blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>          blk_queue_max_segment_size(q, segment_size);
>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>          blk_queue_io_min(q, segment_size);
>>          blk_queue_io_opt(q, segment_size);
>
> I made a similar patch on Friday, investigating blk-mq plugging issue
> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
> AFAIU there is no point in setting to anything bigger since the bios
> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
> 256, this gives is 1M direct I/Os.
Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
request can be merged from multiple bios. We can see the below function:
ll_back_merge_fn, ll_front_merge_fn and etc.
And I test in kernel 3.18 use this patch, and do:
echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
We use systemtap to trace the request size, It is upto 4M.

Thanks.
---------
juncheng bai
>
> Thanks,
>
>                  Ilya
> --
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[Ilya Dryomov]

On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
<baijuncheng@unitedstack.com> wrote:
>
>
> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>
>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>> <baijuncheng@unitedstack.com> wrote:
>>>
>>>  From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>>>   size of the object
>>>
>>> ensures that the merged size of request can achieve the size of
>>> the object.
>>> when merge a bio to request or merge a request to request, the
>>> sum of the segment number of the current request and the segment
>>> number of the bio is not greater than the max segments of the request,
>>> so the max size of request is 512k if the max segments of request is
>>> BLK_MAX_SEGMENTS.
>>>
>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>> ---
>>>   drivers/block/rbd.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>> index 0a54c58..dec6045 100644
>>> --- a/drivers/block/rbd.c
>>> +++ b/drivers/block/rbd.c
>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>> *rbd_dev)
>>>          segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>          blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>          blk_queue_max_segment_size(q, segment_size);
>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>          blk_queue_io_min(q, segment_size);
>>>          blk_queue_io_opt(q, segment_size);
>>
>>
>> I made a similar patch on Friday, investigating blk-mq plugging issue
>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>> AFAIU there is no point in setting to anything bigger since the bios
>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>> 256, this gives is 1M direct I/Os.
>
> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
> request can be merged from multiple bios. We can see the below function:
> ll_back_merge_fn, ll_front_merge_fn and etc.
> And I test in kernel 3.18 use this patch, and do:
> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
> We use systemtap to trace the request size, It is upto 4M.

Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
should test whatever patches you have with at least 4.0.

Putting that aside, I must be missing something.  You'll get 4M
requests on 3.18 both with your patch and without it, the only
difference would be the size of bios being merged - 512k vs 1M.  Can
you describe your test workload and provide before and after traces?

Thanks,

                Ilya

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/15 22:27, Ilya Dryomov wrote:
> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>>
>>
>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>
>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>> <baijuncheng@unitedstack.com> wrote:
>>>>
>>>>   From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>>>>    size of the object
>>>>
>>>> ensures that the merged size of request can achieve the size of
>>>> the object.
>>>> when merge a bio to request or merge a request to request, the
>>>> sum of the segment number of the current request and the segment
>>>> number of the bio is not greater than the max segments of the request,
>>>> so the max size of request is 512k if the max segments of request is
>>>> BLK_MAX_SEGMENTS.
>>>>
>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>> ---
>>>>    drivers/block/rbd.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>> index 0a54c58..dec6045 100644
>>>> --- a/drivers/block/rbd.c
>>>> +++ b/drivers/block/rbd.c
>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>> *rbd_dev)
>>>>           segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>           blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>>           blk_queue_max_segment_size(q, segment_size);
>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>>           blk_queue_io_min(q, segment_size);
>>>>           blk_queue_io_opt(q, segment_size);
>>>
>>>
>>> I made a similar patch on Friday, investigating blk-mq plugging issue
>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>>> AFAIU there is no point in setting to anything bigger since the bios
>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>> 256, this gives is 1M direct I/Os.
>>
>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>> request can be merged from multiple bios. We can see the below function:
>> ll_back_merge_fn, ll_front_merge_fn and etc.
>> And I test in kernel 3.18 use this patch, and do:
>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>> We use systemtap to trace the request size, It is upto 4M.
>
> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
> should test whatever patches you have with at least 4.0.
>
> Putting that aside, I must be missing something.  You'll get 4M
> requests on 3.18 both with your patch and without it, the only
> difference would be the size of bios being merged - 512k vs 1M.  Can
> you describe your test workload and provide before and after traces?
>
Hi. I update kernel version to 4.0.5. The test information as shown below:
The base information:
03:28:13-root@server-186:~$uname -r
4.0.5

My simple systemtap script:
probe module("rbd").function("rbd_img_request_create")
{
     printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
}

I use dd to execute the test case:
dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct

Case one: Without patch
03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
4096
03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
128

The output of systemtap for nornal data:
offset:0 length:524288
offset:524288 length:524288
offset:1048576 length:524288
offset:1572864 length:524288
offset:2097152 length:524288
offset:2621440 length:524288
offset:3145728 length:524288
offset:3670016 length:524288

Case two:With patch
cat /sys/block/rbd0/queue/max_sectors_kb
4096
03:49:14-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/max_segments
1024
The output of systemtap for nornal data:
offset:0 length:1048576
offset:1048576 length:1048576
offset:2097152 length:1048576
offset:3145728 length:1048576

According to the test, you are right.
Because the blk-mq doesn't use any scheduling policy.
03:52:13-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/scheduler
none

In previous versions of the kernel 4.0, the rbd use the defualt 
scheduler:cfq

So, I think that the blk-mq need to do more?
> Thanks,
>
>                  Ilya
>

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[Ilya Dryomov]

On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
<baijuncheng@unitedstack.com> wrote:
>
>
> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>
>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>> <baijuncheng@unitedstack.com> wrote:
>>>
>>>
>>>
>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>
>>>>
>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>
>>>>>
>>>>>   From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00
>>>>> 2001
>>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>>>>>    size of the object
>>>>>
>>>>> ensures that the merged size of request can achieve the size of
>>>>> the object.
>>>>> when merge a bio to request or merge a request to request, the
>>>>> sum of the segment number of the current request and the segment
>>>>> number of the bio is not greater than the max segments of the request,
>>>>> so the max size of request is 512k if the max segments of request is
>>>>> BLK_MAX_SEGMENTS.
>>>>>
>>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>>> ---
>>>>>    drivers/block/rbd.c | 2 ++
>>>>>    1 file changed, 2 insertions(+)
>>>>>
>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>> index 0a54c58..dec6045 100644
>>>>> --- a/drivers/block/rbd.c
>>>>> +++ b/drivers/block/rbd.c
>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>> *rbd_dev)
>>>>>           segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>           blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>>>           blk_queue_max_segment_size(q, segment_size);
>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>>>           blk_queue_io_min(q, segment_size);
>>>>>           blk_queue_io_opt(q, segment_size);
>>>>
>>>>
>>>>
>>>> I made a similar patch on Friday, investigating blk-mq plugging issue
>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>> 256, this gives is 1M direct I/Os.
>>>
>>>
>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>> request can be merged from multiple bios. We can see the below function:
>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>> And I test in kernel 3.18 use this patch, and do:
>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>> We use systemtap to trace the request size, It is upto 4M.
>>
>>
>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>> should test whatever patches you have with at least 4.0.
>>
>> Putting that aside, I must be missing something.  You'll get 4M
>> requests on 3.18 both with your patch and without it, the only
>> difference would be the size of bios being merged - 512k vs 1M.  Can
>> you describe your test workload and provide before and after traces?
>>
> Hi. I update kernel version to 4.0.5. The test information as shown below:
> The base information:
> 03:28:13-root@server-186:~$uname -r
> 4.0.5
>
> My simple systemtap script:
> probe module("rbd").function("rbd_img_request_create")
> {
>     printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
> }
>
> I use dd to execute the test case:
> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>
> Case one: Without patch
> 03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
> 4096
> 03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
> 128
>
> The output of systemtap for nornal data:
> offset:0 length:524288
> offset:524288 length:524288
> offset:1048576 length:524288
> offset:1572864 length:524288
> offset:2097152 length:524288
> offset:2621440 length:524288
> offset:3145728 length:524288
> offset:3670016 length:524288
>
> Case two:With patch
> cat /sys/block/rbd0/queue/max_sectors_kb
> 4096
> 03:49:14-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/max_segments
> 1024
> The output of systemtap for nornal data:
> offset:0 length:1048576
> offset:1048576 length:1048576
> offset:2097152 length:1048576
> offset:3145728 length:1048576
>
> According to the test, you are right.
> Because the blk-mq doesn't use any scheduling policy.
> 03:52:13-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/scheduler
> none
>
> In previous versions of the kernel 4.0, the rbd use the defualt
> scheduler:cfq
>
> So, I think that the blk-mq need to do more?

There is no scheduler support in blk-mq as of now but your numbers
don't have anything to do with that.  The current behaviour is a result
of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
4M requests with your stap script.

[1] http://article.gmane.org/gmane.linux.kernel/1941750

Thanks,

                Ilya

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/16 16:37, Ilya Dryomov wrote:
> On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>>
>>
>> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>>
>>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>>> <baijuncheng@unitedstack.com> wrote:
>>>>
>>>>
>>>>
>>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>>
>>>>>
>>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>
>>>>>>
>>>>>>    From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00
>>>>>> 2001
>>>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>>>>>>     size of the object
>>>>>>
>>>>>> ensures that the merged size of request can achieve the size of
>>>>>> the object.
>>>>>> when merge a bio to request or merge a request to request, the
>>>>>> sum of the segment number of the current request and the segment
>>>>>> number of the bio is not greater than the max segments of the request,
>>>>>> so the max size of request is 512k if the max segments of request is
>>>>>> BLK_MAX_SEGMENTS.
>>>>>>
>>>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>>>> ---
>>>>>>     drivers/block/rbd.c | 2 ++
>>>>>>     1 file changed, 2 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>> index 0a54c58..dec6045 100644
>>>>>> --- a/drivers/block/rbd.c
>>>>>> +++ b/drivers/block/rbd.c
>>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>>> *rbd_dev)
>>>>>>            segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>>            blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>>>>            blk_queue_max_segment_size(q, segment_size);
>>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>>>>            blk_queue_io_min(q, segment_size);
>>>>>>            blk_queue_io_opt(q, segment_size);
>>>>>
>>>>>
>>>>>
>>>>> I made a similar patch on Friday, investigating blk-mq plugging issue
>>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>>> 256, this gives is 1M direct I/Os.
>>>>
>>>>
>>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>>> request can be merged from multiple bios. We can see the below function:
>>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>>> And I test in kernel 3.18 use this patch, and do:
>>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>>> We use systemtap to trace the request size, It is upto 4M.
>>>
>>>
>>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>>> should test whatever patches you have with at least 4.0.
>>>
>>> Putting that aside, I must be missing something.  You'll get 4M
>>> requests on 3.18 both with your patch and without it, the only
>>> difference would be the size of bios being merged - 512k vs 1M.  Can
>>> you describe your test workload and provide before and after traces?
>>>
>> Hi. I update kernel version to 4.0.5. The test information as shown below:
>> The base information:
>> 03:28:13-root@server-186:~$uname -r
>> 4.0.5
>>
>> My simple systemtap script:
>> probe module("rbd").function("rbd_img_request_create")
>> {
>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>> }
>>
>> I use dd to execute the test case:
>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>>
>> Case one: Without patch
>> 03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>> 4096
>> 03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>> 128
>>
>> The output of systemtap for nornal data:
>> offset:0 length:524288
>> offset:524288 length:524288
>> offset:1048576 length:524288
>> offset:1572864 length:524288
>> offset:2097152 length:524288
>> offset:2621440 length:524288
>> offset:3145728 length:524288
>> offset:3670016 length:524288
>>
>> Case two:With patch
>> cat /sys/block/rbd0/queue/max_sectors_kb
>> 4096
>> 03:49:14-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/max_segments
>> 1024
>> The output of systemtap for nornal data:
>> offset:0 length:1048576
>> offset:1048576 length:1048576
>> offset:2097152 length:1048576
>> offset:3145728 length:1048576
>>
>> According to the test, you are right.
>> Because the blk-mq doesn't use any scheduling policy.
>> 03:52:13-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/scheduler
>> none
>>
>> In previous versions of the kernel 4.0, the rbd use the defualt
>> scheduler:cfq
>>
>> So, I think that the blk-mq need to do more?
>
> There is no scheduler support in blk-mq as of now but your numbers
> don't have anything to do with that.  The current behaviour is a result
> of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
> 4M requests with your stap script.
>
> [1] http://article.gmane.org/gmane.linux.kernel/1941750
>
Hi.
First, Let's look at the result in the kernel version 3.18
The function blk_limits_max_hw_sectors different implemention between 
3.18 and 4.0+. We need do:
echo 4094 >/sys/block/rbd0/queue/max_sectors_kb

The rbd device information:
11:13:18-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
4094
11:15:28-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
1024

The test command:
dd if=/dev/zero of=/dev/rbd0 bs=4M count=1

The simple stap script:
probe module("rbd").function("rbd_img_request_create")
{
     printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
}

The output from stap:
offset:0 length:4190208
offset:21474770944 length:4096

Second, thanks for your patch [1].
I use the patch [1], and recompile the kernel.
The test information as shown below:
12:26:12-root@server-186:$cat /sys/block/rbd0/queue/max_segments
1024
12:26:23-root@server-186:$cat /sys/block/rbd0/queue/max_sectors_kb
4096

The test command:
dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct

The simple systemtap script:
probe module("rbd").function("rbd_img_request_create")
{
     printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
}

The output of systemtap for nornal data:
offset:0 length:4194304
offset:4194304 length:4194304
offset:21474770944 length:4096

So, I think that the max_segments of request_limits should be divide the
object size by PAGE_SIZE.

[1] http://article.gmane.org/gmane.linux.kernel/1941750

Thanks.
----
juncheng bai

> Thanks,
>
>                  Ilya
> --
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[Ilya Dryomov]

On Tue, Jun 16, 2015 at 2:57 PM, juncheng bai
<baijuncheng@unitedstack.com> wrote:
>
>
> On 2015/6/16 16:37, Ilya Dryomov wrote:
>>
>> On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
>> <baijuncheng@unitedstack.com> wrote:
>>>
>>>
>>>
>>> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>>>
>>>>
>>>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>    From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00
>>>>>>> 2001
>>>>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to
>>>>>>> the
>>>>>>>     size of the object
>>>>>>>
>>>>>>> ensures that the merged size of request can achieve the size of
>>>>>>> the object.
>>>>>>> when merge a bio to request or merge a request to request, the
>>>>>>> sum of the segment number of the current request and the segment
>>>>>>> number of the bio is not greater than the max segments of the
>>>>>>> request,
>>>>>>> so the max size of request is 512k if the max segments of request is
>>>>>>> BLK_MAX_SEGMENTS.
>>>>>>>
>>>>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>> ---
>>>>>>>     drivers/block/rbd.c | 2 ++
>>>>>>>     1 file changed, 2 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>>> index 0a54c58..dec6045 100644
>>>>>>> --- a/drivers/block/rbd.c
>>>>>>> +++ b/drivers/block/rbd.c
>>>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>>>> *rbd_dev)
>>>>>>>            segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>>>            blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>>>>>            blk_queue_max_segment_size(q, segment_size);
>>>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>>>>>            blk_queue_io_min(q, segment_size);
>>>>>>>            blk_queue_io_opt(q, segment_size);
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> I made a similar patch on Friday, investigating blk-mq plugging issue
>>>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>>>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>>>> 256, this gives is 1M direct I/Os.
>>>>>
>>>>>
>>>>>
>>>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>>>> request can be merged from multiple bios. We can see the below
>>>>> function:
>>>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>>>> And I test in kernel 3.18 use this patch, and do:
>>>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>>>> We use systemtap to trace the request size, It is upto 4M.
>>>>
>>>>
>>>>
>>>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>>>> should test whatever patches you have with at least 4.0.
>>>>
>>>> Putting that aside, I must be missing something.  You'll get 4M
>>>> requests on 3.18 both with your patch and without it, the only
>>>> difference would be the size of bios being merged - 512k vs 1M.  Can
>>>> you describe your test workload and provide before and after traces?
>>>>
>>> Hi. I update kernel version to 4.0.5. The test information as shown
>>> below:
>>> The base information:
>>> 03:28:13-root@server-186:~$uname -r
>>> 4.0.5
>>>
>>> My simple systemtap script:
>>> probe module("rbd").function("rbd_img_request_create")
>>> {
>>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>> }
>>>
>>> I use dd to execute the test case:
>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>>>
>>> Case one: Without patch
>>> 03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>> 4096
>>> 03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>>> 128
>>>
>>> The output of systemtap for nornal data:
>>> offset:0 length:524288
>>> offset:524288 length:524288
>>> offset:1048576 length:524288
>>> offset:1572864 length:524288
>>> offset:2097152 length:524288
>>> offset:2621440 length:524288
>>> offset:3145728 length:524288
>>> offset:3670016 length:524288
>>>
>>> Case two:With patch
>>> cat /sys/block/rbd0/queue/max_sectors_kb
>>> 4096
>>> 03:49:14-root@server-186:linux-4.0.5$cat
>>> /sys/block/rbd0/queue/max_segments
>>> 1024
>>> The output of systemtap for nornal data:
>>> offset:0 length:1048576
>>> offset:1048576 length:1048576
>>> offset:2097152 length:1048576
>>> offset:3145728 length:1048576
>>>
>>> According to the test, you are right.
>>> Because the blk-mq doesn't use any scheduling policy.
>>> 03:52:13-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/scheduler
>>> none
>>>
>>> In previous versions of the kernel 4.0, the rbd use the defualt
>>> scheduler:cfq
>>>
>>> So, I think that the blk-mq need to do more?
>>
>>
>> There is no scheduler support in blk-mq as of now but your numbers
>> don't have anything to do with that.  The current behaviour is a result
>> of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
>> 4M requests with your stap script.
>>
>> [1] http://article.gmane.org/gmane.linux.kernel/1941750
>>
> Hi.
> First, Let's look at the result in the kernel version 3.18
> The function blk_limits_max_hw_sectors different implemention between 3.18
> and 4.0+. We need do:
> echo 4094 >/sys/block/rbd0/queue/max_sectors_kb
>
> The rbd device information:
> 11:13:18-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
> 4094
> 11:15:28-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
> 1024
>
> The test command:
> dd if=/dev/zero of=/dev/rbd0 bs=4M count=1
>
> The simple stap script:
> probe module("rbd").function("rbd_img_request_create")
> {
>     printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
> }
>
> The output from stap:
> offset:0 length:4190208
> offset:21474770944 length:4096
>
> Second, thanks for your patch [1].
> I use the patch [1], and recompile the kernel.
> The test information as shown below:
> 12:26:12-root@server-186:$cat /sys/block/rbd0/queue/max_segments
> 1024
> 12:26:23-root@server-186:$cat /sys/block/rbd0/queue/max_sectors_kb
> 4096
>
> The test command:
> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct
>
> The simple systemtap script:
> probe module("rbd").function("rbd_img_request_create")
> {
>     printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
> }
>
> The output of systemtap for nornal data:
> offset:0 length:4194304
> offset:4194304 length:4194304
> offset:21474770944 length:4096

Sorry, I fail to see the purpose of the above tests.  The test commands
differ, the kernels differ and it looks like you had your patch applied
for both tests.  What I'm trying to get you to do is to show me some
data that will back your claim (which your patch is based on):

>
> So, I think that the max_segments of request_limits should be divide the
> object size by PAGE_SIZE.

For that you need to use the same kernel and run the same workload.
The only difference should be whether your patch is applied or not.
I still think that setting rbd max_segments to anything above
BIO_MAX_PAGES is bogus, but I'd be happy to be shown wrong on that
since that would mean better performance, at least in some
workloads.

Thanks,

                Ilya

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/16 21:30, Ilya Dryomov wrote:
> On Tue, Jun 16, 2015 at 2:57 PM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>>
>>
>> On 2015/6/16 16:37, Ilya Dryomov wrote:
>>>
>>> On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
>>> <baijuncheng@unitedstack.com> wrote:
>>>>
>>>>
>>>>
>>>> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>>>>
>>>>>
>>>>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>     From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00
>>>>>>>> 2001
>>>>>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to
>>>>>>>> the
>>>>>>>>      size of the object
>>>>>>>>
>>>>>>>> ensures that the merged size of request can achieve the size of
>>>>>>>> the object.
>>>>>>>> when merge a bio to request or merge a request to request, the
>>>>>>>> sum of the segment number of the current request and the segment
>>>>>>>> number of the bio is not greater than the max segments of the
>>>>>>>> request,
>>>>>>>> so the max size of request is 512k if the max segments of request is
>>>>>>>> BLK_MAX_SEGMENTS.
>>>>>>>>
>>>>>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>>> ---
>>>>>>>>      drivers/block/rbd.c | 2 ++
>>>>>>>>      1 file changed, 2 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>>>> index 0a54c58..dec6045 100644
>>>>>>>> --- a/drivers/block/rbd.c
>>>>>>>> +++ b/drivers/block/rbd.c
>>>>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>>>>> *rbd_dev)
>>>>>>>>             segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>>>>             blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>>>>>>             blk_queue_max_segment_size(q, segment_size);
>>>>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>>>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>>>>>>             blk_queue_io_min(q, segment_size);
>>>>>>>>             blk_queue_io_opt(q, segment_size);
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I made a similar patch on Friday, investigating blk-mq plugging issue
>>>>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>>>>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>>>>> 256, this gives is 1M direct I/Os.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>>>>> request can be merged from multiple bios. We can see the below
>>>>>> function:
>>>>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>>>>> And I test in kernel 3.18 use this patch, and do:
>>>>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>>>>> We use systemtap to trace the request size, It is upto 4M.
>>>>>
>>>>>
>>>>>
>>>>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>>>>> should test whatever patches you have with at least 4.0.
>>>>>
>>>>> Putting that aside, I must be missing something.  You'll get 4M
>>>>> requests on 3.18 both with your patch and without it, the only
>>>>> difference would be the size of bios being merged - 512k vs 1M.  Can
>>>>> you describe your test workload and provide before and after traces?
>>>>>
>>>> Hi. I update kernel version to 4.0.5. The test information as shown
>>>> below:
>>>> The base information:
>>>> 03:28:13-root@server-186:~$uname -r
>>>> 4.0.5
>>>>
>>>> My simple systemtap script:
>>>> probe module("rbd").function("rbd_img_request_create")
>>>> {
>>>>       printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>>> }
>>>>
>>>> I use dd to execute the test case:
>>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>>>>
>>>> Case one: Without patch
>>>> 03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>>> 4096
>>>> 03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>>>> 128
>>>>
>>>> The output of systemtap for nornal data:
>>>> offset:0 length:524288
>>>> offset:524288 length:524288
>>>> offset:1048576 length:524288
>>>> offset:1572864 length:524288
>>>> offset:2097152 length:524288
>>>> offset:2621440 length:524288
>>>> offset:3145728 length:524288
>>>> offset:3670016 length:524288
>>>>
>>>> Case two:With patch
>>>> cat /sys/block/rbd0/queue/max_sectors_kb
>>>> 4096
>>>> 03:49:14-root@server-186:linux-4.0.5$cat
>>>> /sys/block/rbd0/queue/max_segments
>>>> 1024
>>>> The output of systemtap for nornal data:
>>>> offset:0 length:1048576
>>>> offset:1048576 length:1048576
>>>> offset:2097152 length:1048576
>>>> offset:3145728 length:1048576
>>>>
>>>> According to the test, you are right.
>>>> Because the blk-mq doesn't use any scheduling policy.
>>>> 03:52:13-root@server-186:linux-4.0.5$cat /sys/block/rbd0/queue/scheduler
>>>> none
>>>>
>>>> In previous versions of the kernel 4.0, the rbd use the defualt
>>>> scheduler:cfq
>>>>
>>>> So, I think that the blk-mq need to do more?
>>>
>>>
>>> There is no scheduler support in blk-mq as of now but your numbers
>>> don't have anything to do with that.  The current behaviour is a result
>>> of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
>>> 4M requests with your stap script.
>>>
>>> [1] http://article.gmane.org/gmane.linux.kernel/1941750
>>>
>> Hi.
>> First, Let's look at the result in the kernel version 3.18
>> The function blk_limits_max_hw_sectors different implemention between 3.18
>> and 4.0+. We need do:
>> echo 4094 >/sys/block/rbd0/queue/max_sectors_kb
>>
>> The rbd device information:
>> 11:13:18-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>> 4094
>> 11:15:28-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>> 1024
>>
>> The test command:
>> dd if=/dev/zero of=/dev/rbd0 bs=4M count=1
>>
>> The simple stap script:
>> probe module("rbd").function("rbd_img_request_create")
>> {
>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>> }
>>
>> The output from stap:
>> offset:0 length:4190208
>> offset:21474770944 length:4096
>>
>> Second, thanks for your patch [1].
>> I use the patch [1], and recompile the kernel.
>> The test information as shown below:
>> 12:26:12-root@server-186:$cat /sys/block/rbd0/queue/max_segments
>> 1024
>> 12:26:23-root@server-186:$cat /sys/block/rbd0/queue/max_sectors_kb
>> 4096
>>
>> The test command:
>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct
>>
>> The simple systemtap script:
>> probe module("rbd").function("rbd_img_request_create")
>> {
>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>> }
>>
>> The output of systemtap for nornal data:
>> offset:0 length:4194304
>> offset:4194304 length:4194304
>> offset:21474770944 length:4096
>
> Sorry, I fail to see the purpose of the above tests.  The test commands
> differ, the kernels differ and it looks like you had your patch applied
> for both tests.  What I'm trying to get you to do is to show me some
> data that will back your claim (which your patch is based on):
>
>>
>> So, I think that the max_segments of request_limits should be divide the
>> object size by PAGE_SIZE.
>
> For that you need to use the same kernel and run the same workload.
> The only difference should be whether your patch is applied or not.
> I still think that setting rbd max_segments to anything above
> BIO_MAX_PAGES is bogus, but I'd be happy to be shown wrong on that
> since that would mean better performance, at least in some
> workloads.
>
Hi.
For cloned image, it will avoid doing copyup if the request size is
equal to the object size, I think that it is the key effect of this
patch.
The big request would result in overtime if the ceph backend is busy
or the network bandwidth is too low.
I suggest that add a module parameter to control the value which
decided by the user settings.

Thanks.
----
juncheng bai

> Thanks,
>
>                  Ilya
>

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[Ilya Dryomov]

On Tue, Jun 16, 2015 at 5:14 PM, juncheng bai
<baijuncheng@unitedstack.com> wrote:
>
>
> On 2015/6/16 21:30, Ilya Dryomov wrote:
>>
>> On Tue, Jun 16, 2015 at 2:57 PM, juncheng bai
>> <baijuncheng@unitedstack.com> wrote:
>>>
>>>
>>>
>>> On 2015/6/16 16:37, Ilya Dryomov wrote:
>>>>
>>>>
>>>> On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>     From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17
>>>>>>>>> 00:00:00
>>>>>>>>> 2001
>>>>>>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to
>>>>>>>>> the
>>>>>>>>>      size of the object
>>>>>>>>>
>>>>>>>>> ensures that the merged size of request can achieve the size of
>>>>>>>>> the object.
>>>>>>>>> when merge a bio to request or merge a request to request, the
>>>>>>>>> sum of the segment number of the current request and the segment
>>>>>>>>> number of the bio is not greater than the max segments of the
>>>>>>>>> request,
>>>>>>>>> so the max size of request is 512k if the max segments of request
>>>>>>>>> is
>>>>>>>>> BLK_MAX_SEGMENTS.
>>>>>>>>>
>>>>>>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>>>> ---
>>>>>>>>>      drivers/block/rbd.c | 2 ++
>>>>>>>>>      1 file changed, 2 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>>>>> index 0a54c58..dec6045 100644
>>>>>>>>> --- a/drivers/block/rbd.c
>>>>>>>>> +++ b/drivers/block/rbd.c
>>>>>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>>>>>> *rbd_dev)
>>>>>>>>>             segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>>>>>             blk_queue_max_hw_sectors(q, segment_size /
>>>>>>>>> SECTOR_SIZE);
>>>>>>>>>             blk_queue_max_segment_size(q, segment_size);
>>>>>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>>>>>> +               blk_queue_max_segments(q, segment_size /
>>>>>>>>> PAGE_SIZE);
>>>>>>>>>             blk_queue_io_min(q, segment_size);
>>>>>>>>>             blk_queue_io_opt(q, segment_size);
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> I made a similar patch on Friday, investigating blk-mq plugging
>>>>>>>> issue
>>>>>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally
>>>>>>>> -
>>>>>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>>>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>>>>>> 256, this gives is 1M direct I/Os.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>>>>>> request can be merged from multiple bios. We can see the below
>>>>>>> function:
>>>>>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>>>>>> And I test in kernel 3.18 use this patch, and do:
>>>>>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>>>>>> We use systemtap to trace the request size, It is upto 4M.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>>>>>> should test whatever patches you have with at least 4.0.
>>>>>>
>>>>>> Putting that aside, I must be missing something.  You'll get 4M
>>>>>> requests on 3.18 both with your patch and without it, the only
>>>>>> difference would be the size of bios being merged - 512k vs 1M.  Can
>>>>>> you describe your test workload and provide before and after traces?
>>>>>>
>>>>> Hi. I update kernel version to 4.0.5. The test information as shown
>>>>> below:
>>>>> The base information:
>>>>> 03:28:13-root@server-186:~$uname -r
>>>>> 4.0.5
>>>>>
>>>>> My simple systemtap script:
>>>>> probe module("rbd").function("rbd_img_request_create")
>>>>> {
>>>>>       printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>>>> }
>>>>>
>>>>> I use dd to execute the test case:
>>>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>>>>>
>>>>> Case one: Without patch
>>>>> 03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>>>> 4096
>>>>> 03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>>>>> 128
>>>>>
>>>>> The output of systemtap for nornal data:
>>>>> offset:0 length:524288
>>>>> offset:524288 length:524288
>>>>> offset:1048576 length:524288
>>>>> offset:1572864 length:524288
>>>>> offset:2097152 length:524288
>>>>> offset:2621440 length:524288
>>>>> offset:3145728 length:524288
>>>>> offset:3670016 length:524288
>>>>>
>>>>> Case two:With patch
>>>>> cat /sys/block/rbd0/queue/max_sectors_kb
>>>>> 4096
>>>>> 03:49:14-root@server-186:linux-4.0.5$cat
>>>>> /sys/block/rbd0/queue/max_segments
>>>>> 1024
>>>>> The output of systemtap for nornal data:
>>>>> offset:0 length:1048576
>>>>> offset:1048576 length:1048576
>>>>> offset:2097152 length:1048576
>>>>> offset:3145728 length:1048576
>>>>>
>>>>> According to the test, you are right.
>>>>> Because the blk-mq doesn't use any scheduling policy.
>>>>> 03:52:13-root@server-186:linux-4.0.5$cat
>>>>> /sys/block/rbd0/queue/scheduler
>>>>> none
>>>>>
>>>>> In previous versions of the kernel 4.0, the rbd use the defualt
>>>>> scheduler:cfq
>>>>>
>>>>> So, I think that the blk-mq need to do more?
>>>>
>>>>
>>>>
>>>> There is no scheduler support in blk-mq as of now but your numbers
>>>> don't have anything to do with that.  The current behaviour is a result
>>>> of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
>>>> 4M requests with your stap script.
>>>>
>>>> [1] http://article.gmane.org/gmane.linux.kernel/1941750
>>>>
>>> Hi.
>>> First, Let's look at the result in the kernel version 3.18
>>> The function blk_limits_max_hw_sectors different implemention between
>>> 3.18
>>> and 4.0+. We need do:
>>> echo 4094 >/sys/block/rbd0/queue/max_sectors_kb
>>>
>>> The rbd device information:
>>> 11:13:18-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>> 4094
>>> 11:15:28-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>>> 1024
>>>
>>> The test command:
>>> dd if=/dev/zero of=/dev/rbd0 bs=4M count=1
>>>
>>> The simple stap script:
>>> probe module("rbd").function("rbd_img_request_create")
>>> {
>>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>> }
>>>
>>> The output from stap:
>>> offset:0 length:4190208
>>> offset:21474770944 length:4096
>>>
>>> Second, thanks for your patch [1].
>>> I use the patch [1], and recompile the kernel.
>>> The test information as shown below:
>>> 12:26:12-root@server-186:$cat /sys/block/rbd0/queue/max_segments
>>> 1024
>>> 12:26:23-root@server-186:$cat /sys/block/rbd0/queue/max_sectors_kb
>>> 4096
>>>
>>> The test command:
>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct
>>>
>>> The simple systemtap script:
>>> probe module("rbd").function("rbd_img_request_create")
>>> {
>>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>> }
>>>
>>> The output of systemtap for nornal data:
>>> offset:0 length:4194304
>>> offset:4194304 length:4194304
>>> offset:21474770944 length:4096
>>
>>
>> Sorry, I fail to see the purpose of the above tests.  The test commands
>> differ, the kernels differ and it looks like you had your patch applied
>> for both tests.  What I'm trying to get you to do is to show me some
>> data that will back your claim (which your patch is based on):
>>
>>>
>>> So, I think that the max_segments of request_limits should be divide the
>>> object size by PAGE_SIZE.
>>
>>
>> For that you need to use the same kernel and run the same workload.
>> The only difference should be whether your patch is applied or not.
>> I still think that setting rbd max_segments to anything above
>> BIO_MAX_PAGES is bogus, but I'd be happy to be shown wrong on that
>> since that would mean better performance, at least in some
>> workloads.
>>
> Hi.
> For cloned image, it will avoid doing copyup if the request size is
> equal to the object size, I think that it is the key effect of this
> patch.
> The big request would result in overtime if the ceph backend is busy
> or the network bandwidth is too low.

You are right, but then again: we get rbd object size sized requests
even with the default max_segments.  This is true for both < 4.0 and
>= 4.0 kernels (with the plugging fix applied).

> I suggest that add a module parameter to control the value which
> decided by the user settings.

A module parameter for what exactly?

Thanks,

                Ilya

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/16 23:51, Ilya Dryomov wrote:
> On Tue, Jun 16, 2015 at 5:14 PM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>>
>>
>> On 2015/6/16 21:30, Ilya Dryomov wrote:
>>>
>>> On Tue, Jun 16, 2015 at 2:57 PM, juncheng bai
>>> <baijuncheng@unitedstack.com> wrote:
>>>>
>>>>
>>>>
>>>> On 2015/6/16 16:37, Ilya Dryomov wrote:
>>>>>
>>>>>
>>>>> On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>>>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>>>>>>> <baijuncheng@unitedstack.com> wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>      From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17
>>>>>>>>>> 00:00:00
>>>>>>>>>> 2001
>>>>>>>>>> From: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>>>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to
>>>>>>>>>> the
>>>>>>>>>>       size of the object
>>>>>>>>>>
>>>>>>>>>> ensures that the merged size of request can achieve the size of
>>>>>>>>>> the object.
>>>>>>>>>> when merge a bio to request or merge a request to request, the
>>>>>>>>>> sum of the segment number of the current request and the segment
>>>>>>>>>> number of the bio is not greater than the max segments of the
>>>>>>>>>> request,
>>>>>>>>>> so the max size of request is 512k if the max segments of request
>>>>>>>>>> is
>>>>>>>>>> BLK_MAX_SEGMENTS.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: juncheng bai <baijuncheng@unitedstack.com>
>>>>>>>>>> ---
>>>>>>>>>>       drivers/block/rbd.c | 2 ++
>>>>>>>>>>       1 file changed, 2 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>>>>>> index 0a54c58..dec6045 100644
>>>>>>>>>> --- a/drivers/block/rbd.c
>>>>>>>>>> +++ b/drivers/block/rbd.c
>>>>>>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>>>>>>> *rbd_dev)
>>>>>>>>>>              segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>>>>>>              blk_queue_max_hw_sectors(q, segment_size /
>>>>>>>>>> SECTOR_SIZE);
>>>>>>>>>>              blk_queue_max_segment_size(q, segment_size);
>>>>>>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>>>>>>> +               blk_queue_max_segments(q, segment_size /
>>>>>>>>>> PAGE_SIZE);
>>>>>>>>>>              blk_queue_io_min(q, segment_size);
>>>>>>>>>>              blk_queue_io_opt(q, segment_size);
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I made a similar patch on Friday, investigating blk-mq plugging
>>>>>>>>> issue
>>>>>>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally
>>>>>>>>> -
>>>>>>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>>>>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>>>>>>> 256, this gives is 1M direct I/Os.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>>>>>>> request can be merged from multiple bios. We can see the below
>>>>>>>> function:
>>>>>>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>>>>>>> And I test in kernel 3.18 use this patch, and do:
>>>>>>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>>>>>>> We use systemtap to trace the request size, It is upto 4M.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>>>>>>> should test whatever patches you have with at least 4.0.
>>>>>>>
>>>>>>> Putting that aside, I must be missing something.  You'll get 4M
>>>>>>> requests on 3.18 both with your patch and without it, the only
>>>>>>> difference would be the size of bios being merged - 512k vs 1M.  Can
>>>>>>> you describe your test workload and provide before and after traces?
>>>>>>>
>>>>>> Hi. I update kernel version to 4.0.5. The test information as shown
>>>>>> below:
>>>>>> The base information:
>>>>>> 03:28:13-root@server-186:~$uname -r
>>>>>> 4.0.5
>>>>>>
>>>>>> My simple systemtap script:
>>>>>> probe module("rbd").function("rbd_img_request_create")
>>>>>> {
>>>>>>        printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>>>>> }
>>>>>>
>>>>>> I use dd to execute the test case:
>>>>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>>>>>>
>>>>>> Case one: Without patch
>>>>>> 03:30:23-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>>>>> 4096
>>>>>> 03:30:35-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>>>>>> 128
>>>>>>
>>>>>> The output of systemtap for nornal data:
>>>>>> offset:0 length:524288
>>>>>> offset:524288 length:524288
>>>>>> offset:1048576 length:524288
>>>>>> offset:1572864 length:524288
>>>>>> offset:2097152 length:524288
>>>>>> offset:2621440 length:524288
>>>>>> offset:3145728 length:524288
>>>>>> offset:3670016 length:524288
>>>>>>
>>>>>> Case two:With patch
>>>>>> cat /sys/block/rbd0/queue/max_sectors_kb
>>>>>> 4096
>>>>>> 03:49:14-root@server-186:linux-4.0.5$cat
>>>>>> /sys/block/rbd0/queue/max_segments
>>>>>> 1024
>>>>>> The output of systemtap for nornal data:
>>>>>> offset:0 length:1048576
>>>>>> offset:1048576 length:1048576
>>>>>> offset:2097152 length:1048576
>>>>>> offset:3145728 length:1048576
>>>>>>
>>>>>> According to the test, you are right.
>>>>>> Because the blk-mq doesn't use any scheduling policy.
>>>>>> 03:52:13-root@server-186:linux-4.0.5$cat
>>>>>> /sys/block/rbd0/queue/scheduler
>>>>>> none
>>>>>>
>>>>>> In previous versions of the kernel 4.0, the rbd use the defualt
>>>>>> scheduler:cfq
>>>>>>
>>>>>> So, I think that the blk-mq need to do more?
>>>>>
>>>>>
>>>>>
>>>>> There is no scheduler support in blk-mq as of now but your numbers
>>>>> don't have anything to do with that.  The current behaviour is a result
>>>>> of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
>>>>> 4M requests with your stap script.
>>>>>
>>>>> [1] http://article.gmane.org/gmane.linux.kernel/1941750
>>>>>
>>>> Hi.
>>>> First, Let's look at the result in the kernel version 3.18
>>>> The function blk_limits_max_hw_sectors different implemention between
>>>> 3.18
>>>> and 4.0+. We need do:
>>>> echo 4094 >/sys/block/rbd0/queue/max_sectors_kb
>>>>
>>>> The rbd device information:
>>>> 11:13:18-root@server-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>>> 4094
>>>> 11:15:28-root@server-186:~$cat /sys/block/rbd0/queue/max_segments
>>>> 1024
>>>>
>>>> The test command:
>>>> dd if=/dev/zero of=/dev/rbd0 bs=4M count=1
>>>>
>>>> The simple stap script:
>>>> probe module("rbd").function("rbd_img_request_create")
>>>> {
>>>>       printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>>> }
>>>>
>>>> The output from stap:
>>>> offset:0 length:4190208
>>>> offset:21474770944 length:4096
>>>>
>>>> Second, thanks for your patch [1].
>>>> I use the patch [1], and recompile the kernel.
>>>> The test information as shown below:
>>>> 12:26:12-root@server-186:$cat /sys/block/rbd0/queue/max_segments
>>>> 1024
>>>> 12:26:23-root@server-186:$cat /sys/block/rbd0/queue/max_sectors_kb
>>>> 4096
>>>>
>>>> The test command:
>>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct
>>>>
>>>> The simple systemtap script:
>>>> probe module("rbd").function("rbd_img_request_create")
>>>> {
>>>>       printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>>> }
>>>>
>>>> The output of systemtap for nornal data:
>>>> offset:0 length:4194304
>>>> offset:4194304 length:4194304
>>>> offset:21474770944 length:4096
>>>
>>>
>>> Sorry, I fail to see the purpose of the above tests.  The test commands
>>> differ, the kernels differ and it looks like you had your patch applied
>>> for both tests.  What I'm trying to get you to do is to show me some
>>> data that will back your claim (which your patch is based on):
>>>
>>>>
>>>> So, I think that the max_segments of request_limits should be divide the
>>>> object size by PAGE_SIZE.
>>>
>>>
>>> For that you need to use the same kernel and run the same workload.
>>> The only difference should be whether your patch is applied or not.
>>> I still think that setting rbd max_segments to anything above
>>> BIO_MAX_PAGES is bogus, but I'd be happy to be shown wrong on that
>>> since that would mean better performance, at least in some
>>> workloads.
>>>
>> Hi.
>> For cloned image, it will avoid doing copyup if the request size is
>> equal to the object size, I think that it is the key effect of this
>> patch.
>> The big request would result in overtime if the ceph backend is busy
>> or the network bandwidth is too low.
>
> You are right, but then again: we get rbd object size sized requests
> even with the default max_segments.  This is true for both < 4.0 and
>> = 4.0 kernels (with the plugging fix applied).
Hi.
Yeah, you are right, use the default max_segments, the request size can
be the object size, because the bi_phys_segments of bio could be 
recount, there's just a possibility.

I want to fully understand the bi_phys_segments, hope you can give me 
some information, thanks.

The test information as shown below:
The systemtap script:
global greq=0;
probe kernel.function("bio_attempt_back_merge")
{
     greq=pointer_arg(2);
}

probe kernel.function("bio_attempt_back_merge").return
{
     printf("after req addr:%p req segments:%d req offset:%lu req 
length:%lu\n",
         greq,
         @cast(greq, "request")->nr_phys_segments,
         @cast(greq, "request")->__sector * 512,
         @cast(greq, "request")->__data_len);
}

probe kernel.function("blk_mq_start_request")
{
     printf("req addr:%p nr_phys_segments:%d, offset:%lu len:%lu\n",
         pointer_arg(1),
         @cast(pointer_arg(1), "request")->nr_phys_segments,
         @cast(pointer_arg(1), "request")->__sector * 512,
         @cast(pointer_arg(1), "request")->__data_len);
}

Test command:
dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct seek=100

Cast one:
blk_queue_max_segments(q, 256);

The output of stap:
after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 
req length:2097152
after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 
req length:2097152
after req addr:0xffff880ff60a0a80 req segments:186 req offset:421527552 
req length:1048576
req addr:0xffff880ff60a08c0 nr_phys_segments:73, offset:419430400 
len:2097152
req addr:0xffff880ff60a0a80 nr_phys_segments:186, offset:421527552 
len:1048576
req addr:0xffff880ff60a0c40 nr_phys_segments:232, offset:422576128 
len:1048576

after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 
req length:2097152
after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 
req length:2097152
after req addr:0xffff880ff60a0e00 req segments:186 req offset:425721856 
req length:1048576
req addr:0xffff880ff60a0c40 nr_phys_segments:73, offset:423624704 
len:2097152
req addr:0xffff880ff60a0e00 nr_phys_segments:186, offset:425721856 
len:1048576
req addr:0xffff880ff60a0fc0 nr_phys_segments:232, offset:426770432 
len:1048576

Case two:
blk_queue_max_segments(q, segment_size / PAGE_SIZE);

The output of stap:
after req addr:0xffff88101c9a0000 req segments:478 req offset:419430400 
req length:4194304
req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:419430400 
len:4194304

after req addr:0xffff88101c9a0000 req segments:478 req offset:423624704 
req length:4194304
req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:423624704 
len:4194304

1.Based on the setting of max_sectors and max_segments, decides the
   size of a request.
2.We have already set max_sectors to an object's size, so we should try
   to ensure that a request to the size as possible as merge bio.

Thanks.
----
juncheng bai
>
>> I suggest that add a module parameter to control the value which
>> decided by the user settings.
>
> A module parameter for what exactly?
It likes the single_major of module rbd.
>
> Thanks,
>
>                  Ilya
>

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[Ilya Dryomov]

On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai
<baijuncheng@unitedstack.com> wrote:
> Hi.
> Yeah, you are right, use the default max_segments, the request size can
> be the object size, because the bi_phys_segments of bio could be recount,
> there's just a possibility.
>
> I want to fully understand the bi_phys_segments, hope you can give me some
> information, thanks.
>
> The test information as shown below:
> The systemtap script:
> global greq=0;
> probe kernel.function("bio_attempt_back_merge")
> {
>     greq=pointer_arg(2);
> }
>
> probe kernel.function("bio_attempt_back_merge").return
> {
>     printf("after req addr:%p req segments:%d req offset:%lu req
> length:%lu\n",
>         greq,
>         @cast(greq, "request")->nr_phys_segments,
>         @cast(greq, "request")->__sector * 512,
>         @cast(greq, "request")->__data_len);
> }
>
> probe kernel.function("blk_mq_start_request")
> {
>     printf("req addr:%p nr_phys_segments:%d, offset:%lu len:%lu\n",
>         pointer_arg(1),
>         @cast(pointer_arg(1), "request")->nr_phys_segments,
>         @cast(pointer_arg(1), "request")->__sector * 512,
>         @cast(pointer_arg(1), "request")->__data_len);
> }
>
> Test command:
> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct seek=100
>
> Cast one:
> blk_queue_max_segments(q, 256);
>
> The output of stap:
> after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 req
> length:2097152
> after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 req
> length:2097152
> after req addr:0xffff880ff60a0a80 req segments:186 req offset:421527552 req
> length:1048576
> req addr:0xffff880ff60a08c0 nr_phys_segments:73, offset:419430400
> len:2097152
> req addr:0xffff880ff60a0a80 nr_phys_segments:186, offset:421527552
> len:1048576
> req addr:0xffff880ff60a0c40 nr_phys_segments:232, offset:422576128
> len:1048576
>
> after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 req
> length:2097152
> after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 req
> length:2097152
> after req addr:0xffff880ff60a0e00 req segments:186 req offset:425721856 req
> length:1048576
> req addr:0xffff880ff60a0c40 nr_phys_segments:73, offset:423624704
> len:2097152
> req addr:0xffff880ff60a0e00 nr_phys_segments:186, offset:425721856
> len:1048576
> req addr:0xffff880ff60a0fc0 nr_phys_segments:232, offset:426770432
> len:1048576
>
> Case two:
> blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>
> The output of stap:
> after req addr:0xffff88101c9a0000 req segments:478 req offset:419430400 req
> length:4194304
> req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:419430400
> len:4194304
>
> after req addr:0xffff88101c9a0000 req segments:478 req offset:423624704 req
> length:4194304
> req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:423624704
> len:4194304
>
> 1.Based on the setting of max_sectors and max_segments, decides the
>   size of a request.
> 2.We have already set max_sectors to an object's size, so we should try
>   to ensure that a request to the size as possible as merge bio.

Yeah, I also tried to explain this in the commit description [1].
Initially I had BIO_MAX_PAGES in there, and realistically I still think
it's enough for most cases, but discussion with you made me consider
readv/writev case and so I changed it in my patch to max_hw_sectors
(i.e. segment_size / SECTOR_SIZE) - this ensures that max_segments will
never be a limiting factor even in theory.

[1] https://github.com/ceph/ceph-client/commit/2d8006795564fbc0fa68d75758f605fe9f7a108e

Thanks,

                Ilya

Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/17 16:24, Ilya Dryomov wrote:
> On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>> Hi.
>> Yeah, you are right, use the default max_segments, the request size can
>> be the object size, because the bi_phys_segments of bio could be recount,
>> there's just a possibility.
>>
>> I want to fully understand the bi_phys_segments, hope you can give me some
>> information, thanks.
>>
>> The test information as shown below:
>> The systemtap script:
>> global greq=0;
>> probe kernel.function("bio_attempt_back_merge")
>> {
>>      greq=pointer_arg(2);
>> }
>>
>> probe kernel.function("bio_attempt_back_merge").return
>> {
>>      printf("after req addr:%p req segments:%d req offset:%lu req
>> length:%lu\n",
>>          greq,
>>          @cast(greq, "request")->nr_phys_segments,
>>          @cast(greq, "request")->__sector * 512,
>>          @cast(greq, "request")->__data_len);
>> }
>>
>> probe kernel.function("blk_mq_start_request")
>> {
>>      printf("req addr:%p nr_phys_segments:%d, offset:%lu len:%lu\n",
>>          pointer_arg(1),
>>          @cast(pointer_arg(1), "request")->nr_phys_segments,
>>          @cast(pointer_arg(1), "request")->__sector * 512,
>>          @cast(pointer_arg(1), "request")->__data_len);
>> }
>>
>> Test command:
>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct seek=100
>>
>> Cast one:
>> blk_queue_max_segments(q, 256);
>>
>> The output of stap:
>> after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 req
>> length:2097152
>> after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 req
>> length:2097152
>> after req addr:0xffff880ff60a0a80 req segments:186 req offset:421527552 req
>> length:1048576
>> req addr:0xffff880ff60a08c0 nr_phys_segments:73, offset:419430400
>> len:2097152
>> req addr:0xffff880ff60a0a80 nr_phys_segments:186, offset:421527552
>> len:1048576
>> req addr:0xffff880ff60a0c40 nr_phys_segments:232, offset:422576128
>> len:1048576
>>
>> after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 req
>> length:2097152
>> after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 req
>> length:2097152
>> after req addr:0xffff880ff60a0e00 req segments:186 req offset:425721856 req
>> length:1048576
>> req addr:0xffff880ff60a0c40 nr_phys_segments:73, offset:423624704
>> len:2097152
>> req addr:0xffff880ff60a0e00 nr_phys_segments:186, offset:425721856
>> len:1048576
>> req addr:0xffff880ff60a0fc0 nr_phys_segments:232, offset:426770432
>> len:1048576
>>
>> Case two:
>> blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>
>> The output of stap:
>> after req addr:0xffff88101c9a0000 req segments:478 req offset:419430400 req
>> length:4194304
>> req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:419430400
>> len:4194304
>>
>> after req addr:0xffff88101c9a0000 req segments:478 req offset:423624704 req
>> length:4194304
>> req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:423624704
>> len:4194304
>>
>> 1.Based on the setting of max_sectors and max_segments, decides the
>>    size of a request.
>> 2.We have already set max_sectors to an object's size, so we should try
>>    to ensure that a request to the size as possible as merge bio.
>
> Yeah, I also tried to explain this in the commit description [1].
> Initially I had BIO_MAX_PAGES in there, and realistically I still think
> it's enough for most cases, but discussion with you made me consider
> readv/writev case and so I changed it in my patch to max_hw_sectors
> (i.e. segment_size / SECTOR_SIZE) - this ensures that max_segments will
> never be a limiting factor even in theory.
>
> [1] https://github.com/ceph/ceph-client/commit/2d8006795564fbc0fa68d75758f605fe9f7a108e
>
> Thanks,
>
>                  Ilya
>
Yeah, I agree with you.

Thanks.
----
juncheng bai