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

[Paolo Valente]

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.

Thanks,
Paolo

> 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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