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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