Re: CFQ idling kills I/O performance on ext4 with blkio cgroup controller

["Srivatsa S. Bhat" <srivatsa@csail.mit.edu>]

On 5/20/19 11:23 PM, Paolo Valente wrote:
> 
> 
>> Il giorno 21 mag 2019, alle ore 00:45, Srivatsa S. Bhat <srivatsa@csail.mit.edu> ha scritto:
>>
>> On 5/20/19 3:19 AM, Paolo Valente wrote:
>>>
>>>
>>>> Il giorno 18 mag 2019, alle ore 22:50, Srivatsa S. Bhat <srivatsa@csail.mit.edu> ha scritto:
>>>>
>>>> On 5/18/19 11:39 AM, Paolo Valente wrote:
>>>>> I've addressed these issues in my last batch of improvements for BFQ,
>>>>> which landed in the upcoming 5.2. If you give it a try, and still see
>>>>> the problem, then I'll be glad to reproduce it, and hopefully fix it
>>>>> for you.
>>>>>
>>>>
>>>> Hi Paolo,
>>>>
>>>> Thank you for looking into this!
>>>>
>>>> I just tried current mainline at commit 72cf0b07, but unfortunately
>>>> didn't see any improvement:
>>>>
>>>> dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflag=dsync
>>>>
>>>> With mq-deadline, I get:
>>>>
>>>> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 3.90981 s, 1.3 MB/s
>>>>
>>>> With bfq, I get:
>>>> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 84.8216 s, 60.4 kB/s
>>>>
>>>
>>> Hi Srivatsa,
>>> thanks for reproducing this on mainline.  I seem to have reproduced a
>>> bonsai-tree version of this issue.  Before digging into the block
>>> trace, I'd like to ask you for some feedback.
>>>
>>> First, in my test, the total throughput of the disk happens to be
>>> about 20 times as high as that enjoyed by dd, regardless of the I/O
>>> scheduler.  I guess this massive overhead is normal with dsync, but
>>> I'd like know whether it is about the same on your side.  This will
>>> help me understand whether I'll actually be analyzing about the same
>>> problem as yours.
>>>
>>
>> Do you mean to say the throughput obtained by dd'ing directly to the
>> block device (bypassing the filesystem)?
> 
> No no, I mean simply what follows.
> 
> 1) in one terminal:
> [root@localhost tmp]# dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflag=dsync
> 10000+0 record dentro
> 10000+0 record fuori
> 5120000 bytes (5,1 MB, 4,9 MiB) copied, 14,6892 s, 349 kB/s
> 
> 2) In a second terminal, while the dd is in progress in the first
> terminal:
> $ iostat -tmd /dev/sda 3
> Linux 5.1.0+ (localhost.localdomain) 	20/05/2019 	_x86_64_	(2 CPU)
> 
> ...
> 20/05/2019 11:40:17
> Device             tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
> sda            2288,00         0,00         9,77          0         29
> 
> 20/05/2019 11:40:20
> Device             tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
> sda            2325,33         0,00         9,93          0         29
> 
> 20/05/2019 11:40:23
> Device             tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
> sda            2351,33         0,00        10,05          0         30
> ...
> 
> As you can see, the overall throughput (~10 MB/s) is more than 20
> times as high as the dd throughput (~350 KB/s).  But the dd is the
> only source of I/O.
> 
> Do you also see such a huge difference?
> 
Ah, I see what you mean. Yes, I get a huge difference as well:

I/O scheduler    dd throughput    Total throughput (via iostat)
-------------    -------------    -----------------------------

mq-deadline
    or              1.6 MB/s               50 MB/s (30x)
  kyber

   bfq               60 KB/s                1 MB/s (16x)


Regards,
Srivatsa
VMware Photon OS