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

[Paolo Valente]

> Il giorno 21 mag 2019, alle ore 18:21, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> 
> 
> 
>> Il giorno 21 mag 2019, alle ore 15:20, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
>> 
>> 
>> 
>>> Il giorno 21 mag 2019, alle ore 13:25, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
>>> 
>>> 
>>> 
>>>> Il giorno 20 mag 2019, alle ore 12:19, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
>>>> 
>>>> 
>>>> 
>>>>> 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.
>>> 
>>> 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@xxxxxxxxxxxxx> 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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