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    Subject[Regression] High latency when doing large I/O

    A long standing I/O regression (since 2.6.18, still there today) has hit
    Slashdot recently :

    I've taken a trace reproducing the wrong behavior on my machine and I
    think it's getting us somewhere.

    LTTng 0.83, kernel 2.6.28
    Machine : Intel Xeon E5405 dual quad-core, 16GB ram
    (just created a new block-trace.c LTTng probe which is not released yet.
    It basically replaces blktrace)

    echo 3 > /proc/sys/vm/drop_caches

    lttctl -C -w /tmp/trace -o -o channel.block.bufnum=64 trace

    dd if=/dev/zero of=/tmp/newfile bs=1M count=1M
    cp -ax music /tmp (copying 1.1GB of mp3)

    ls (takes 15 seconds to get the directory listing !)

    lttctl -D trace

    I looked at the trace (especially at the ls surroundings), and bash is
    waiting for a few seconds for I/O in the exec system call (to exec ls).

    While this happens, we have dd doing lots and lots of bio_queue. There
    is a bio_backmerge after each bio_queue event. This is reasonable,
    because dd is writing to a contiguous file.

    However, I wonder if this is not the actual problem. We have dd which
    has the head request in the elevator request queue. It is progressing
    steadily by plugging/unplugging the device periodically and gets its
    work done. However, because requests are being dequeued at the same
    rate others are being merged, I suspect it stays at the top of the queue
    and does not let the other unrelated requests run.

    There is a test in the blk-merge.c which makes sure that merged requests
    do not get bigger than a certain size. However, if the request is
    steadily dequeued, I think this test is not doing anything.

    If you are interested in looking at the trace I've taken, I could
    provide it.

    Does that make sense ?


    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68

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