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Date:   Tue, 21 May 2019 15:20:12 +0200
From:   Paolo Valente <>
To:     "Srivatsa S. Bhat" <>
        linux-block <>,,,
        kernel list <>,
        Jens Axboe <>, Jan Kara <>,,,,,
Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup

> Il giorno 21 mag 2019, alle ore 13:25, Paolo Valente <> ha scritto:
>> Il giorno 20 mag 2019, alle ore 12:19, Paolo Valente <> ha scritto:
>>> Il giorno 18 mag 2019, alle ore 22:50, Srivatsa S. Bhat <> 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.
> Hi again Srivatsa,
> I've analyzed the trace, and I've found the cause of the loss of
> throughput in on my side.  To find out whether it is the same cause as
> on your side, I've prepared a script that executes your test and takes
> a trace during the test.  If ok for you, could you please
> - change the value for the DEVS parameter in the attached script, if
>  needed
> - execute the script
> - send me the trace file that the script will leave in your working
> dir

Sorry, I forgot to add that I also need you to, first, apply the
attached patch (it will make BFQ generate the log I need).


Download attachment "0001-block-bfq-add-logs-and-BUG_ONs.patch.gz" of type "application/x-gzip" (27285 bytes)

> Looking forward to your trace,
> Paolo
> <>
>> 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.
>> Second, the commands I used follow.  Do they implement your test case
>> correctly?
>> [root@...alhost tmp]# mkdir /sys/fs/cgroup/blkio/testgrp
>> [root@...alhost tmp]# echo $BASHPID > /sys/fs/cgroup/blkio/testgrp/cgroup.procs
>> [root@...alhost tmp]# cat /sys/block/sda/queue/scheduler
>> [mq-deadline] bfq none
>> [root@...alhost 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@...alhost tmp]# echo bfq > /sys/block/sda/queue/scheduler
>> [root@...alhost 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
>> Thanks,
>> Paolo
>>> Please let me know if any more info about my setup might be helpful.
>>> Thank you!
>>> Regards,
>>> Srivatsa
>>> VMware Photon OS
>>>>> Il giorno 18 mag 2019, alle ore 00:16, Srivatsa S. Bhat <> ha scritto:
>>>>> Hi,
>>>>> One of my colleagues noticed upto 10x - 30x drop in I/O throughput
>>>>> running the following command, with the CFQ I/O scheduler:
>>>>> dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflags=dsync
>>>>> Throughput with CFQ: 60 KB/s
>>>>> Throughput with noop or deadline: 1.5 MB/s - 2 MB/s
>>>>> I spent some time looking into it and found that this is caused by the
>>>>> undesirable interaction between 4 different components:
>>>>> - blkio cgroup controller enabled
>>>>> - ext4 with the jbd2 kthread running in the root blkio cgroup
>>>>> - dd running on ext4, in any other blkio cgroup than that of jbd2
>>>>> - CFQ I/O scheduler with defaults for slice_idle and group_idle
>>>>> When docker is enabled, systemd creates a blkio cgroup called
>>>>> system.slice to run system services (and docker) under it, and a
>>>>> separate blkio cgroup called user.slice for user processes. So, when
>>>>> dd is invoked, it runs under user.slice.
>>>>> The dd command above includes the dsync flag, which performs an
>>>>> fdatasync after every write to the output file. Since dd is writing to
>>>>> a file on ext4, jbd2 will be active, committing transactions
>>>>> corresponding to those fdatasync requests from dd. (In other words, dd
>>>>> depends on jdb2, in order to make forward progress). But jdb2 being a
>>>>> kernel thread, runs in the root blkio cgroup, as opposed to dd, which
>>>>> runs under user.slice.
>>>>> Now, if the I/O scheduler in use for the underlying block device is
>>>>> CFQ, then its inter-queue/inter-group idling takes effect (via the
>>>>> slice_idle and group_idle parameters, both of which default to 8ms).
>>>>> Therefore, everytime CFQ switches between processing requests from dd
>>>>> vs jbd2, this 8ms idle time is injected, which slows down the overall
>>>>> throughput tremendously!
>>>>> To verify this theory, I tried various experiments, and in all cases,
>>>>> the 4 pre-conditions mentioned above were necessary to reproduce this
>>>>> performance drop. For example, if I used an XFS filesystem (which
>>>>> doesn't use a separate kthread like jbd2 for journaling), or if I dd'ed
>>>>> directly to a block device, I couldn't reproduce the performance
>>>>> issue. Similarly, running dd in the root blkio cgroup (where jbd2
>>>>> runs) also gets full performance; as does using the noop or deadline
>>>>> I/O schedulers; or even CFQ itself, with slice_idle and group_idle set
>>>>> to zero.
>>>>> These results were reproduced on a Linux VM (kernel v4.19) on ESXi,
>>>>> both with virtualized storage as well as with disk pass-through,
>>>>> backed by a rotational hard disk in both cases. The same problem was
>>>>> also seen with the BFQ I/O scheduler in kernel v5.1.
>>>>> Searching for any earlier discussions of this problem, I found an old
>>>>> thread on LKML that encountered this behavior [1], as well as a docker
>>>>> github issue [2] with similar symptoms (mentioned later in the
>>>>> thread).
>>>>> So, I'm curious to know if this is a well-understood problem and if
>>>>> anybody has any thoughts on how to fix it.
>>>>> Thank you very much!
>>>>> [1].
>>>>> [2].
>>>>> Regards,
>>>>> Srivatsa

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