Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Michael S. Tsirkin]

On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
> This change makes ring buffer reclaim threshold num_free configurable
> for better performance, while it's hard coded as 1/2 * queue now.
> According to our test with qemu + dpdk, packet dropping happens when
> the guest is not able to provide free buffer in avail ring timely.
> Smaller value of num_free does decrease the number of packet dropping
> during our test as it makes virtio_net reclaim buffer earlier.
> 
> At least, we should leave the value changeable to user while the
> default value as 1/2 * queue is kept.
> 
> Signed-off-by: jiangkidd <jiangkidd@hotmail.com>

That would be one reason, but I suspect it's not the
true one. If you need more buffer due to jitter
then just increase the queue size. Would be cleaner.


However are you sure this is the reason for
packet drops? Do you see them dropped by dpdk
due to lack of space in the ring? As opposed to
by guest?


> ---
>  drivers/net/virtio_net.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 0d4115c9e20b..bc190dec6084 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -26,6 +26,9 @@
>  static int napi_weight = NAPI_POLL_WEIGHT;
>  module_param(napi_weight, int, 0444);
>  
> +static int min_numfree;
> +module_param(min_numfree, int, 0444);
> +
>  static bool csum = true, gso = true, napi_tx;
>  module_param(csum, bool, 0444);
>  module_param(gso, bool, 0444);
> @@ -1315,6 +1318,9 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  	void *buf;
>  	int i;
>  
> +	if (!min_numfree)
> +		min_numfree = virtqueue_get_vring_size(rq->vq) / 2;
> +
>  	if (!vi->big_packets || vi->mergeable_rx_bufs) {
>  		void *ctx;
>  
> @@ -1331,7 +1337,7 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  		}
>  	}
>  
> -	if (rq->vq->num_free > virtqueue_get_vring_size(rq->vq) / 2) {
> +	if (rq->vq->num_free > min_numfree) {
>  		if (!try_fill_recv(vi, rq, GFP_ATOMIC))
>  			schedule_delayed_work(&vi->refill, 0);
>  	}
> -- 
> 2.11.0

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Jason Wang]

On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
>> This change makes ring buffer reclaim threshold num_free configurable
>> for better performance, while it's hard coded as 1/2 * queue now.
>> According to our test with qemu + dpdk, packet dropping happens when
>> the guest is not able to provide free buffer in avail ring timely.
>> Smaller value of num_free does decrease the number of packet dropping
>> during our test as it makes virtio_net reclaim buffer earlier.
>>
>> At least, we should leave the value changeable to user while the
>> default value as 1/2 * queue is kept.
>>
>> Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
> That would be one reason, but I suspect it's not the
> true one. If you need more buffer due to jitter
> then just increase the queue size. Would be cleaner.
>
>
> However are you sure this is the reason for
> packet drops? Do you see them dropped by dpdk
> due to lack of space in the ring? As opposed to
> by guest?
>
>

Besides those, this patch depends on the user to choose a suitable 
threshold which is not good. You need either a good value with 
demonstrated numbers or something smarter.

Thanks

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Michael S. Tsirkin]

On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
> 
> On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
> > On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
> > > This change makes ring buffer reclaim threshold num_free configurable
> > > for better performance, while it's hard coded as 1/2 * queue now.
> > > According to our test with qemu + dpdk, packet dropping happens when
> > > the guest is not able to provide free buffer in avail ring timely.
> > > Smaller value of num_free does decrease the number of packet dropping
> > > during our test as it makes virtio_net reclaim buffer earlier.
> > > 
> > > At least, we should leave the value changeable to user while the
> > > default value as 1/2 * queue is kept.
> > > 
> > > Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
> > That would be one reason, but I suspect it's not the
> > true one. If you need more buffer due to jitter
> > then just increase the queue size. Would be cleaner.
> > 
> > 
> > However are you sure this is the reason for
> > packet drops? Do you see them dropped by dpdk
> > due to lack of space in the ring? As opposed to
> > by guest?
> > 
> > 
> 
> Besides those, this patch depends on the user to choose a suitable threshold
> which is not good. You need either a good value with demonstrated numbers or
> something smarter.
> 
> Thanks

I do however think that we have a problem right now: try_fill_recv can
take up a long time during which net stack does not run at all. Imagine
a 1K queue - we are talking 512 packets. That's exceessive.  napi poll
weight solves a similar problem, so it might make sense to cap this at
napi_poll_weight.

Which will allow tweaking it through a module parameter as a
side effect :) Maybe just do NAPI_POLL_WEIGHT.

Need to be careful though: queues can also be small and I don't think we
want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
Definitely must not exceed the full queue size.

-- 
MST

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Michael S. Tsirkin]

On Thu, Jul 18, 2019 at 10:42:47AM -0400, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
> > 
> > On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
> > > On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
> > > > This change makes ring buffer reclaim threshold num_free configurable
> > > > for better performance, while it's hard coded as 1/2 * queue now.
> > > > According to our test with qemu + dpdk, packet dropping happens when
> > > > the guest is not able to provide free buffer in avail ring timely.
> > > > Smaller value of num_free does decrease the number of packet dropping
> > > > during our test as it makes virtio_net reclaim buffer earlier.
> > > > 
> > > > At least, we should leave the value changeable to user while the
> > > > default value as 1/2 * queue is kept.
> > > > 
> > > > Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
> > > That would be one reason, but I suspect it's not the
> > > true one. If you need more buffer due to jitter
> > > then just increase the queue size. Would be cleaner.
> > > 
> > > 
> > > However are you sure this is the reason for
> > > packet drops? Do you see them dropped by dpdk
> > > due to lack of space in the ring? As opposed to
> > > by guest?
> > > 
> > > 
> > 
> > Besides those, this patch depends on the user to choose a suitable threshold
> > which is not good. You need either a good value with demonstrated numbers or
> > something smarter.
> > 
> > Thanks
> 
> I do however think that we have a problem right now: try_fill_recv can
> take up a long time during which net stack does not run at all. Imagine
> a 1K queue - we are talking 512 packets. That's exceessive.  napi poll
> weight solves a similar problem, so it might make sense to cap this at
> napi_poll_weight.
> 
> Which will allow tweaking it through a module parameter as a
> side effect :) Maybe just do NAPI_POLL_WEIGHT.

Or maybe NAPI_POLL_WEIGHT/2 like we do at half the queue ;). Please
experiment, measure performance and let the list know

> Need to be careful though: queues can also be small and I don't think we
> want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
> Definitely must not exceed the full queue size.
> 
> -- 
> MST

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Jason Wang]

On 2019/7/18 下午10:43, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 10:42:47AM -0400, Michael S. Tsirkin wrote:
>> On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
>>> On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
>>>> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
>>>>> This change makes ring buffer reclaim threshold num_free configurable
>>>>> for better performance, while it's hard coded as 1/2 * queue now.
>>>>> According to our test with qemu + dpdk, packet dropping happens when
>>>>> the guest is not able to provide free buffer in avail ring timely.
>>>>> Smaller value of num_free does decrease the number of packet dropping
>>>>> during our test as it makes virtio_net reclaim buffer earlier.
>>>>>
>>>>> At least, we should leave the value changeable to user while the
>>>>> default value as 1/2 * queue is kept.
>>>>>
>>>>> Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
>>>> That would be one reason, but I suspect it's not the
>>>> true one. If you need more buffer due to jitter
>>>> then just increase the queue size. Would be cleaner.
>>>>
>>>>
>>>> However are you sure this is the reason for
>>>> packet drops? Do you see them dropped by dpdk
>>>> due to lack of space in the ring? As opposed to
>>>> by guest?
>>>>
>>>>
>>> Besides those, this patch depends on the user to choose a suitable threshold
>>> which is not good. You need either a good value with demonstrated numbers or
>>> something smarter.
>>>
>>> Thanks
>> I do however think that we have a problem right now: try_fill_recv can
>> take up a long time during which net stack does not run at all. Imagine
>> a 1K queue - we are talking 512 packets. That's exceessive.


Yes, we will starve a fast host in this case.


>>    napi poll
>> weight solves a similar problem, so it might make sense to cap this at
>> napi_poll_weight.
>>
>> Which will allow tweaking it through a module parameter as a
>> side effect :) Maybe just do NAPI_POLL_WEIGHT.
> Or maybe NAPI_POLL_WEIGHT/2 like we do at half the queue ;). Please
> experiment, measure performance and let the list know
>
>> Need to be careful though: queues can also be small and I don't think we
>> want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
>> Definitely must not exceed the full queue size.


Looking at intel, it uses 16 and i40e uses 32.  It looks to me 
NAPI_POLL_WEIGHT/2 is better.

Jiang, want to try that and post a new patch?

Thanks


>>
>> -- 
>> MST

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Michael S. Tsirkin]

On Fri, Jul 19, 2019 at 03:31:29PM +0000, 冉 jiang wrote:
> 
> On 2019/7/19 22:29, Jiang wrote:
> >
> > On 2019/7/19 10:36, Jason Wang wrote:
> >>
> >> On 2019/7/18 下午10:43, Michael S. Tsirkin wrote:
> >>> On Thu, Jul 18, 2019 at 10:42:47AM -0400, Michael S. Tsirkin wrote:
> >>>> On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
> >>>>> On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
> >>>>>> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
> >>>>>>> This change makes ring buffer reclaim threshold num_free 
> >>>>>>> configurable
> >>>>>>> for better performance, while it's hard coded as 1/2 * queue now.
> >>>>>>> According to our test with qemu + dpdk, packet dropping happens 
> >>>>>>> when
> >>>>>>> the guest is not able to provide free buffer in avail ring timely.
> >>>>>>> Smaller value of num_free does decrease the number of packet 
> >>>>>>> dropping
> >>>>>>> during our test as it makes virtio_net reclaim buffer earlier.
> >>>>>>>
> >>>>>>> At least, we should leave the value changeable to user while the
> >>>>>>> default value as 1/2 * queue is kept.
> >>>>>>>
> >>>>>>> Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
> >>>>>> That would be one reason, but I suspect it's not the
> >>>>>> true one. If you need more buffer due to jitter
> >>>>>> then just increase the queue size. Would be cleaner.
> >>>>>>
> >>>>>>
> >>>>>> However are you sure this is the reason for
> >>>>>> packet drops? Do you see them dropped by dpdk
> >>>>>> due to lack of space in the ring? As opposed to
> >>>>>> by guest?
> >>>>>>
> >>>>>>
> >>>>> Besides those, this patch depends on the user to choose a suitable 
> >>>>> threshold
> >>>>> which is not good. You need either a good value with demonstrated 
> >>>>> numbers or
> >>>>> something smarter.
> >>>>>
> >>>>> Thanks
> >>>> I do however think that we have a problem right now: try_fill_recv can
> >>>> take up a long time during which net stack does not run at all. 
> >>>> Imagine
> >>>> a 1K queue - we are talking 512 packets. That's exceessive.
> >>
> >>
> >> Yes, we will starve a fast host in this case.
> >>
> >>
> >>>>    napi poll
> >>>> weight solves a similar problem, so it might make sense to cap this at
> >>>> napi_poll_weight.
> >>>>
> >>>> Which will allow tweaking it through a module parameter as a
> >>>> side effect :) Maybe just do NAPI_POLL_WEIGHT.
> >>> Or maybe NAPI_POLL_WEIGHT/2 like we do at half the queue ;). Please
> >>> experiment, measure performance and let the list know
> >>>
> >>>> Need to be careful though: queues can also be small and I don't 
> >>>> think we
> >>>> want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
> >>>> Definitely must not exceed the full queue size.
> >>
> >>
> >> Looking at intel, it uses 16 and i40e uses 32.  It looks to me 
> >> NAPI_POLL_WEIGHT/2 is better.
> >>
> >> Jiang, want to try that and post a new patch?
> >>
> >> Thanks
> >>
> >>
> >>>>
> >>>> -- 
> >>>> MST
> >
> > We did have completed several rounds of test with setting the value to 
> > budget (64 as the default value). It does improve a lot with pps is 
> > below 400pps for a single stream. Let me consolidate the data and will 
> > send it soon. Actually, we are confident that it runs out of free 
> > buffer in avail ring when packet dropping happens with below systemtap:
> >
> > Just a snippet:
> >
> > probe module("virtio_ring").function("virtqueue_get_buf")
> > {
> >     x = (@cast($_vq, "vring_virtqueue")->vring->used->idx)- 
> > (@cast($_vq, "vring_virtqueue")->last_used_idx) ---> we use this one 
> > to verify if the queue is full, which means guest is not able to take 
> > buffer from the queue timely
> >
> >     if (x<0 && (x+65535)<4096)
> >         x = x+65535
> >
> >     if((x==1024) && @cast($_vq, "vring_virtqueue")->vq->callback == 
> > callback_addr)
> >         netrxcount[x] <<< gettimeofday_s()
> > }
> >
> >
> > probe module("virtio_ring").function("virtqueue_add_inbuf")
> > {
> >     y = (@cast($vq, "vring_virtqueue")->vring->avail->idx)- 
> > (@cast($vq, "vring_virtqueue")->vring->used->idx) ---> we use this one 
> > to verify if we run out of free buffer in avail ring
> >     if (y<0 && (y+65535)<4096)
> >         y = y+65535
> >
> >     if(@2=="debugon")
> >     {
> >         if(y==0 && @cast($vq, "vring_virtqueue")->vq->callback == 
> > callback_addr)
> >         {
> >             netrxfreecount[y] <<< gettimeofday_s()
> >
> >             printf("no avail ring left seen, printing most recent 5 
> > num free, vq: %lx, current index: %d\n", $vq, recentfreecount)
> >             for(i=recentfreecount; i!=((recentfreecount+4) % 5); 
> > i=((i+1) % 5))
> >             {
> >                 printf("index: %d, num free: %d\n", i, recentfree[$vq, 
> > i])
> >             }
> >
> >             printf("index: %d, num free: %d\n", i, recentfree[$vq, i])
> >             //exit()
> >         }
> >     }
> > }
> >
> >
> > probe 
> > module("virtio_net").statement("virtnet_receive@drivers/net/virtio_net.c:732")
> > {
> >     recentfreecount++
> >     recentfreecount = recentfreecount % 5
> >     recentfree[$rq->vq, recentfreecount] = $rq->vq->num_free ---> 
> > record the num_free for the last 5 calls to virtnet_receive, so we can 
> > see if lowering the bar helps.
> > }
> >
> >
> > Here is the result:
> >
> > no avail ring left seen, printing most recent 5 num free, vq: 
> > ffff9c13c1200000, current index: 1
> > index: 1, num free: 561
> > index: 2, num free: 305
> > index: 3, num free: 369
> > index: 4, num free: 433
> > index: 0, num free: 497
> > no avail ring left seen, printing most recent 5 num free, vq: 
> > ffff9c13c1200000, current index: 1
> > index: 1, num free: 543
> > index: 2, num free: 463
> > index: 3, num free: 469
> > index: 4, num free: 476
> > index: 0, num free: 479
> > no avail ring left seen, printing most recent 5 num free, vq: 
> > ffff9c13c1200000, current index: 2
> > index: 2, num free: 555
> > index: 3, num free: 414
> > index: 4, num free: 420
> > index: 0, num free: 427
> > index: 1, num free: 491
> >
> > You can see in the last 4 calls to virtnet_receive before we run out 
> > of free buffer and start to relaim, num_free is quite high. So if we 
> > can do the reclaim earlier, it will certainly help.
> >
> > Meanwhile, the patch I proposed actually keeps the default value as 
> > 1/2 * queue. So the default behavior remains and only leave the 
> > interface to advanced users, who really understands what they are 
> > doing. Also, the best value may vary in different environment. Do you 
> > still think hardcoding this is better option?
> >
> >
> > Jiang
> >
> Here is the snippet from our test result. Test1 was done with default 
> driver with the value of 1/2 * queue, while test2 is with my patch and 
> min_numfree set to 64 (the default budget value). We can see average 
> drop packets do decrease a lot in test2. Let me know if you need the 
> full testing data.
> 
> test1Time    avgDropPackets    test2Time    avgDropPackets    pps
> 
> > 16:21.0    12.295    56:50.4    0    300k
> > 17:19.1    15.244    56:50.4    0    300k
> > 18:17.5    18.789    56:50.4    0    300k
> > 19:15.1    14.208    56:50.4    0    300k
> > 20:13.2    20.818    56:50.4    0.267    300k
> > 21:11.2    12.397    56:50.4    0    300k
> > 22:09.3    12.599    56:50.4    0    300k
> > 23:07.3    15.531    57:48.4    0    300k
> > 24:05.5    13.664    58:46.5    0    300k
> > 25:03.7    13.158    59:44.5    4.73    300k
> > 26:01.1    2.486    00:42.6    0    300k
> > 26:59.1    11.241    01:40.6    0    300k
> > 27:57.2    20.521    02:38.6    0    300k
> > 28:55.2    30.094    03:36.7    0    300k
> > 29:53.3    16.828    04:34.7    0.963    300k
> > 30:51.3    46.916    05:32.8    0    400k
> > 31:49.3    56.214    05:32.8    0    400k
> > 32:47.3    58.69    05:32.8    0    400k
> > 33:45.3    61.486    05:32.8    0    400k
> > 34:43.3    72.175    05:32.8    0.598    400k
> > 35:41.3    56.699    05:32.8    0    400k
> > 36:39.3    61.071    05:32.8    0    400k
> > 37:37.3    43.355    06:30.8    0    400k
> > 38:35.4    44.644    06:30.8    0    400k
> > 39:33.4    72.336    06:30.8    0    400k
> > 40:31.4    70.676    06:30.8    0    400k
> > 41:29.4    108.009    06:30.8    0    400k
> > 42:27.4    65.216    06:30.8    0    400k
> 
> 
> Jiang


OK I find this surprising but I accept what you see.
I'm inclined not to add a tunable and just select
a value ourselves.
I'm also fine with using the napi poll module parameter
which will give you a bit of tunability.

-- 
MST

Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Michael S. Tsirkin]

On Tue, Jul 23, 2019 at 12:05:03PM +0000, 冉 jiang wrote:
> 
> On 2019/7/20 0:13, Michael S. Tsirkin wrote:
> > On Fri, Jul 19, 2019 at 03:31:29PM +0000, 冉 jiang wrote:
> >> On 2019/7/19 22:29, Jiang wrote:
> >>> On 2019/7/19 10:36, Jason Wang wrote:
> >>>> On 2019/7/18 下午10:43, Michael S. Tsirkin wrote:
> >>>>> On Thu, Jul 18, 2019 at 10:42:47AM -0400, Michael S. Tsirkin wrote:
> >>>>>> On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
> >>>>>>> On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
> >>>>>>>> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
> >>>>>>>>> This change makes ring buffer reclaim threshold num_free
> >>>>>>>>> configurable
> >>>>>>>>> for better performance, while it's hard coded as 1/2 * queue now.
> >>>>>>>>> According to our test with qemu + dpdk, packet dropping happens
> >>>>>>>>> when
> >>>>>>>>> the guest is not able to provide free buffer in avail ring timely.
> >>>>>>>>> Smaller value of num_free does decrease the number of packet
> >>>>>>>>> dropping
> >>>>>>>>> during our test as it makes virtio_net reclaim buffer earlier.
> >>>>>>>>>
> >>>>>>>>> At least, we should leave the value changeable to user while the
> >>>>>>>>> default value as 1/2 * queue is kept.
> >>>>>>>>>
> >>>>>>>>> Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
> >>>>>>>> That would be one reason, but I suspect it's not the
> >>>>>>>> true one. If you need more buffer due to jitter
> >>>>>>>> then just increase the queue size. Would be cleaner.
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> However are you sure this is the reason for
> >>>>>>>> packet drops? Do you see them dropped by dpdk
> >>>>>>>> due to lack of space in the ring? As opposed to
> >>>>>>>> by guest?
> >>>>>>>>
> >>>>>>>>
> >>>>>>> Besides those, this patch depends on the user to choose a suitable
> >>>>>>> threshold
> >>>>>>> which is not good. You need either a good value with demonstrated
> >>>>>>> numbers or
> >>>>>>> something smarter.
> >>>>>>>
> >>>>>>> Thanks
> >>>>>> I do however think that we have a problem right now: try_fill_recv can
> >>>>>> take up a long time during which net stack does not run at all.
> >>>>>> Imagine
> >>>>>> a 1K queue - we are talking 512 packets. That's exceessive.
> >>>>
> >>>> Yes, we will starve a fast host in this case.
> >>>>
> >>>>
> >>>>>>     napi poll
> >>>>>> weight solves a similar problem, so it might make sense to cap this at
> >>>>>> napi_poll_weight.
> >>>>>>
> >>>>>> Which will allow tweaking it through a module parameter as a
> >>>>>> side effect :) Maybe just do NAPI_POLL_WEIGHT.
> >>>>> Or maybe NAPI_POLL_WEIGHT/2 like we do at half the queue ;). Please
> >>>>> experiment, measure performance and let the list know
> >>>>>
> >>>>>> Need to be careful though: queues can also be small and I don't
> >>>>>> think we
> >>>>>> want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
> >>>>>> Definitely must not exceed the full queue size.
> >>>>
> >>>> Looking at intel, it uses 16 and i40e uses 32.  It looks to me
> >>>> NAPI_POLL_WEIGHT/2 is better.
> >>>>
> >>>> Jiang, want to try that and post a new patch?
> >>>>
> >>>> Thanks
> >>>>
> >>>>
> >>>>>> -- 
> >>>>>> MST
> >>> We did have completed several rounds of test with setting the value to
> >>> budget (64 as the default value). It does improve a lot with pps is
> >>> below 400pps for a single stream. Let me consolidate the data and will
> >>> send it soon. Actually, we are confident that it runs out of free
> >>> buffer in avail ring when packet dropping happens with below systemtap:
> >>>
> >>> Just a snippet:
> >>>
> >>> probe module("virtio_ring").function("virtqueue_get_buf")
> >>> {
> >>>      x = (@cast($_vq, "vring_virtqueue")->vring->used->idx)-
> >>> (@cast($_vq, "vring_virtqueue")->last_used_idx) ---> we use this one
> >>> to verify if the queue is full, which means guest is not able to take
> >>> buffer from the queue timely
> >>>
> >>>      if (x<0 && (x+65535)<4096)
> >>>          x = x+65535
> >>>
> >>>      if((x==1024) && @cast($_vq, "vring_virtqueue")->vq->callback ==
> >>> callback_addr)
> >>>          netrxcount[x] <<< gettimeofday_s()
> >>> }
> >>>
> >>>
> >>> probe module("virtio_ring").function("virtqueue_add_inbuf")
> >>> {
> >>>      y = (@cast($vq, "vring_virtqueue")->vring->avail->idx)-
> >>> (@cast($vq, "vring_virtqueue")->vring->used->idx) ---> we use this one
> >>> to verify if we run out of free buffer in avail ring
> >>>      if (y<0 && (y+65535)<4096)
> >>>          y = y+65535
> >>>
> >>>      if(@2=="debugon")
> >>>      {
> >>>          if(y==0 && @cast($vq, "vring_virtqueue")->vq->callback ==
> >>> callback_addr)
> >>>          {
> >>>              netrxfreecount[y] <<< gettimeofday_s()
> >>>
> >>>              printf("no avail ring left seen, printing most recent 5
> >>> num free, vq: %lx, current index: %d\n", $vq, recentfreecount)
> >>>              for(i=recentfreecount; i!=((recentfreecount+4) % 5);
> >>> i=((i+1) % 5))
> >>>              {
> >>>                  printf("index: %d, num free: %d\n", i, recentfree[$vq,
> >>> i])
> >>>              }
> >>>
> >>>              printf("index: %d, num free: %d\n", i, recentfree[$vq, i])
> >>>              //exit()
> >>>          }
> >>>      }
> >>> }
> >>>
> >>>
> >>> probe
> >>> module("virtio_net").statement("virtnet_receive@drivers/net/virtio_net.c:732")
> >>> {
> >>>      recentfreecount++
> >>>      recentfreecount = recentfreecount % 5
> >>>      recentfree[$rq->vq, recentfreecount] = $rq->vq->num_free --->
> >>> record the num_free for the last 5 calls to virtnet_receive, so we can
> >>> see if lowering the bar helps.
> >>> }
> >>>
> >>>
> >>> Here is the result:
> >>>
> >>> no avail ring left seen, printing most recent 5 num free, vq:
> >>> ffff9c13c1200000, current index: 1
> >>> index: 1, num free: 561
> >>> index: 2, num free: 305
> >>> index: 3, num free: 369
> >>> index: 4, num free: 433
> >>> index: 0, num free: 497
> >>> no avail ring left seen, printing most recent 5 num free, vq:
> >>> ffff9c13c1200000, current index: 1
> >>> index: 1, num free: 543
> >>> index: 2, num free: 463
> >>> index: 3, num free: 469
> >>> index: 4, num free: 476
> >>> index: 0, num free: 479
> >>> no avail ring left seen, printing most recent 5 num free, vq:
> >>> ffff9c13c1200000, current index: 2
> >>> index: 2, num free: 555
> >>> index: 3, num free: 414
> >>> index: 4, num free: 420
> >>> index: 0, num free: 427
> >>> index: 1, num free: 491
> >>>
> >>> You can see in the last 4 calls to virtnet_receive before we run out
> >>> of free buffer and start to relaim, num_free is quite high. So if we
> >>> can do the reclaim earlier, it will certainly help.
> >>>
> >>> Meanwhile, the patch I proposed actually keeps the default value as
> >>> 1/2 * queue. So the default behavior remains and only leave the
> >>> interface to advanced users, who really understands what they are
> >>> doing. Also, the best value may vary in different environment. Do you
> >>> still think hardcoding this is better option?
> >>>
> >>>
> >>> Jiang
> >>>
> >> Here is the snippet from our test result. Test1 was done with default
> >> driver with the value of 1/2 * queue, while test2 is with my patch and
> >> min_numfree set to 64 (the default budget value). We can see average
> >> drop packets do decrease a lot in test2. Let me know if you need the
> >> full testing data.
> >>
> >> test1Time    avgDropPackets    test2Time    avgDropPackets    pps
> >>
> >>> 16:21.0    12.295    56:50.4    0    300k
> >>> 17:19.1    15.244    56:50.4    0    300k
> >>> 18:17.5    18.789    56:50.4    0    300k
> >>> 19:15.1    14.208    56:50.4    0    300k
> >>> 20:13.2    20.818    56:50.4    0.267    300k
> >>> 21:11.2    12.397    56:50.4    0    300k
> >>> 22:09.3    12.599    56:50.4    0    300k
> >>> 23:07.3    15.531    57:48.4    0    300k
> >>> 24:05.5    13.664    58:46.5    0    300k
> >>> 25:03.7    13.158    59:44.5    4.73    300k
> >>> 26:01.1    2.486    00:42.6    0    300k
> >>> 26:59.1    11.241    01:40.6    0    300k
> >>> 27:57.2    20.521    02:38.6    0    300k
> >>> 28:55.2    30.094    03:36.7    0    300k
> >>> 29:53.3    16.828    04:34.7    0.963    300k
> >>> 30:51.3    46.916    05:32.8    0    400k
> >>> 31:49.3    56.214    05:32.8    0    400k
> >>> 32:47.3    58.69    05:32.8    0    400k
> >>> 33:45.3    61.486    05:32.8    0    400k
> >>> 34:43.3    72.175    05:32.8    0.598    400k
> >>> 35:41.3    56.699    05:32.8    0    400k
> >>> 36:39.3    61.071    05:32.8    0    400k
> >>> 37:37.3    43.355    06:30.8    0    400k
> >>> 38:35.4    44.644    06:30.8    0    400k
> >>> 39:33.4    72.336    06:30.8    0    400k
> >>> 40:31.4    70.676    06:30.8    0    400k
> >>> 41:29.4    108.009    06:30.8    0    400k
> >>> 42:27.4    65.216    06:30.8    0    400k
> >>
> >> Jiang
> >
> > OK I find this surprising but I accept what you see.
> > I'm inclined not to add a tunable and just select
> > a value ourselves.
> > I'm also fine with using the napi poll module parameter
> > which will give you a bit of tunability.
> 
> OK, kindly take a look if you prefer the below code change. I tested 
> budget/2 and the result is almost the same as budget when pps below 
> 400k, but a little better when it goes beyond 400k in my environment.
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> 
> index 0d4115c9e20b..bc08be7925eb 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1331,7 +1331,7 @@ static int virtnet_receive(struct receive_queue 
> *rq, int budget,
>                  }
>          }
> 
> -       if (rq->vq->num_free > virtqueue_get_vring_size(rq->vq) / 2) {
> +       if (rq->vq->num_free > min((unsigned int)budget, 
> virtqueue_get_vring_size(rq->vq)) / 2) {
>                  if (!try_fill_recv(vi, rq, GFP_ATOMIC))
>                          schedule_delayed_work(&vi->refill, 0);
>          }
> 
> 
> Jiang
>

Looks good to me.
Pls post for inclusion in -net.

-- 
MST