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From:   Paolo Valente <paolo.valente@linaro.org>
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Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup
 controller
Date:   Tue, 21 May 2019 08:23:05 +0200
In-Reply-To: <238e14ff-68d1-3b21-a291-28de4f2d77af@csail.mit.edu>
Cc:     linux-fsdevel@vger.kernel.org,
        linux-block <linux-block@vger.kernel.org>,
        linux-ext4@vger.kernel.org, cgroups@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        jmoyer@redhat.com, Theodore Ts'o <tytso@mit.edu>,
        amakhalov@vmware.com, anishs@vmware.com, srivatsab@vmware.com
To:     "Srivatsa S. Bhat" <srivatsa@csail.mit.edu>
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> Il giorno 21 mag 2019, alle ore 00:45, Srivatsa S. Bhat =
<srivatsa@csail.mit.edu> ha scritto:
>=20
> On 5/20/19 3:19 AM, Paolo Valente wrote:
>>=20
>>=20
>>> Il giorno 18 mag 2019, alle ore 22:50, Srivatsa S. Bhat =
<srivatsa@csail.mit.edu> ha scritto:
>>>=20
>>> 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.
>>>>=20
>>>=20
>>> Hi Paolo,
>>>=20
>>> Thank you for looking into this!
>>>=20
>>> I just tried current mainline at commit 72cf0b07, but unfortunately
>>> didn't see any improvement:
>>>=20
>>> dd if=3D/dev/zero of=3D/root/test.img bs=3D512 count=3D10000 =
oflag=3Ddsync
>>>=20
>>> With mq-deadline, I get:
>>>=20
>>> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 3.90981 s, 1.3 MB/s
>>>=20
>>> With bfq, I get:
>>> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 84.8216 s, 60.4 kB/s
>>>=20
>>=20
>> 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.
>>=20
>> 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.
>>=20
>=20
> 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=3D/dev/zero of=3D/root/test.img bs=3D512 =
count=3D10000 oflag=3Ddsync
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=3D512, but much more with a bigger block size =
(achieving
> a max throughput of about 110 MB/s).
>=20
> dd if=3D/dev/zero of=3D/dev/sdc bs=3D512 count=3D10000 conv=3Dfsync
> 10000+0 records in
> 10000+0 records out
> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 0.15257 s, 33.6 MB/s
>=20
> dd if=3D/dev/zero of=3D/dev/sdc bs=3D4k count=3D10000 conv=3Dfsync
> 10000+0 records in
> 10000+0 records out
> 40960000 bytes (41 MB, 39 MiB) copied, 0.395081 s, 104 MB/s
>=20
> I'm testing this on a Toshiba MG03ACA1 (1TB) hard disk.
>=20
>> Second, the commands I used follow.  Do they implement your test case
>> correctly?
>>=20
>> [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=3D/dev/zero of=3D/root/test.img bs=3D512 =
count=3D10000 oflag=3Ddsync
>> 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=3D/dev/zero of=3D/root/test.img bs=3D512 =
count=3D10000 oflag=3Ddsync
>> 10000+0 record dentro
>> 10000+0 record fuori
>> 5120000 bytes (5,1 MB, 4,9 MiB) copied, 20,1953 s, 254 kB/s
>>=20
>=20
> Yes, this is indeed the testcase, although I see a much bigger
> drop in performance with bfq, compared to the results from
> your setup.
>=20
> Regards,
> Srivatsa


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