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

[Paolo Valente]

> Il giorno 21 mag 2019, alle ore 00:45, Srivatsa S. Bhat <srivatsa@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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?

Thanks,
Paolo

> That does give me a 20x
> speedup with bs=512, but much more with a bigger block size (achieving
> a max throughput of about 110 MB/s).
> 
> dd if=/dev/zero of=/dev/sdc bs=512 count=10000 conv=fsync
> 10000+0 records in
> 10000+0 records out
> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 0.15257 s, 33.6 MB/s
> 
> dd if=/dev/zero of=/dev/sdc bs=4k count=10000 conv=fsync
> 10000+0 records in
> 10000+0 records out
> 40960000 bytes (41 MB, 39 MiB) copied, 0.395081 s, 104 MB/s
> 
> I'm testing this on a Toshiba MG03ACA1 (1TB) hard disk.
> 
>> Second, the commands I used follow.  Do they implement your test case
>> correctly?
>> 
>> [root@localhost tmp]# mkdir /sys/fs/cgroup/blkio/testgrp
>> [root@localhost tmp]# echo $BASHPID > /sys/fs/cgroup/blkio/testgrp/cgroup.procs
>> [root@localhost tmp]# cat /sys/block/sda/queue/scheduler
>> [mq-deadline] bfq none
>> [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
>> [root@localhost tmp]# echo bfq > /sys/block/sda/queue/scheduler
>> [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, 20,1953 s, 254 kB/s
>> 
> 
> Yes, this is indeed the testcase, although I see a much bigger
> drop in performance with bfq, compared to the results from
> your setup.
> 
> Regards,
> Srivatsa

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