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SubjectRe: CFQ idling kills I/O performance on ext4 with blkio cgroup controller
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> Il giorno 21 mag 2019, alle ore 18:21, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>
>
>
>> Il giorno 21 mag 2019, alle ore 15:20, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>
>>
>>
>>> Il giorno 21 mag 2019, alle ore 13:25, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>>
>>>
>>>
>>>> Il giorno 20 mag 2019, alle ore 12:19, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>>>
>>>>
>>>>
>>>>> 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.
>>>
>>> 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).
>>
>
> Sorry again :) This time for attaching one more patch. This is
> basically a blind fix attempt, based on what I see in my VM.
>
> So, instead of only sending me a trace, could you please:
> 1) apply this new patch on top of the one I attached in my previous email
> 2) repeat your test and report results

One last thing (I swear!): as you can see from my script, I tested the
case low_latency=0 so far. So please, for the moment, do your test
with low_latency=0. You find the whole path to this parameter in,
e.g., my script.

Thanks,
Paolo

> 3) regardless of whether bfq performance improves, take a trace with
> my script (I've attached a new version that doesn't risk to output an
> annoying error message as the previous one)
>
> Thanks,
> Paolo
>
> <dsync_test.sh><0001-block-bfq-boost-injection.patch.gz>
>
>> Thanks,
>> Paolo
>>
>> <0001-block-bfq-add-logs-and-BUG_ONs.patch.gz>
>>
>>> Looking forward to your trace,
>>> Paolo
>>>
>>> <dsync_test.sh>
>>>> 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@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
>>>>
>>>> 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 <srivatsa@csail.mit.edu> 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]. https://lkml.org/lkml/2015/11/19/359
>>>>>>>
>>>>>>> [2]. https://github.com/moby/moby/issues/21485
>>>>>>> https://github.com/moby/moby/issues/21485#issuecomment-222941103
>>>>>>>
>>>>>>> Regards,
>>>>>>> Srivatsa

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