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
