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 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 >
