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    Subjectsevere jitter experienced with "select()" in linux 2.6.14-rt22
    I have been conduting some jitter tests on 2.6.14-rt22.  (Many thanks
    to Ingo Molnar who has helped in various ways.) I wanted to share the
    results with others and seek comments as to the problems I see and
    whether it is possible to overcome them and how might I go about it.

    My tests are rather simple. I take 1 million samples of the actual
    durations of nanosleep() versus the requested 1000 usec duration.

    a = gettimeofday(); /* measure delta time since last sleep */
    for (i = 0; i < NUM_SAMPLES; i++) { /* one million iterations,
    typically */
    nanosleep(1000000); /* sleep for 1 msec; = 1000 HZ */
    b = gettimeofday(); /* measure delta time since last sleep */
    record (b - a); /* record the measurement in a histogram */
    a = b;

    The histogram of the "nanosleep()" tests can be viewed at:

    The results are excellent with actual sleep durations for
    nanosleep(1000 usec) being:

    minimum: 1020 (usec)
    maximum: 1052
    mean: 1030
    variance: 3.876
    num_points: 1000000

    I then repeated this test by replacing nanosleep(1000 usec) with

    select(0, 0, 0, 0, 1000usec)

    And again measure the observed jitter. The test application is run in
    SCHED_FIFO class at priority 60; the softirq-* processes are run at
    SCHED_FIFO priority 90. In no case does select() return anything other
    than value of zero.

    The select() test exepriences severe jitter. Histogram can be viewed

    and the summary of observed select() sleep duration samples is:

    minimum: 35 (usec)
    maximum: 239525
    mean: 1373.24
    variance: 8.99334e+07
    num_points: 1000000

    The computer is otherwise unloaded and unused and in a small, private
    network. The h/w is a 2.8 GHZ dual Xeon IBM Blade, but I have been
    testing with 2.6.14-rt22 UniProcessor kernel configured with full
    preemption and 1000 HZ clock. The kernel is compiled with "preemption
    latencry tracing" flags but no tracing is enabled during either of
    these two tests.

    Data summry in a table form can be seen here:

    My queries are this:

    1) are these results believable? I like to think that i am careful
    tester, but error in testing is always possible.

    2) Can this jitter in select() be overcome and can select() get as
    good as nanosleep() is now under the "rt" kernel patches?

    3) I have tried to do some latency tracing, but I can't seem to learn
    anything valulabe as to the cause. I have followed the procedure
    outlined by Ingo Molnar, but it is not clear if "dmesg" or
    "/proc/latency_trace" reveal anything of value.

    /proc/latency trace is full of lines such as these:

    <...>-3 0.... 20317us : __down_mutex (rt_run_flush)
    <...>-3 0.... 20317us : __up_mutex_savestate (rt_run_flush)
    <...>-3 0.... 20317us : __down_mutex (rt_run_flush)
    <...>-3 0.... 20317us : __up_mutex_savestate (rt_run_flush)
    <...>-3 0.... 20317us : __down_mutex (rt_run_flush)
    <...>-3 0.... 20317us : __up_mutex_savestate (rt_run_flush)
    <...>-3 0.... 20318us : __down_mutex (rt_run_flush)
    <...>-3 0.... 20318us : __up_mutex_savestate (rt_run_flush)
    <...>-3 0.... 20318us : __down_mutex (rt_run_flush)
    <...>-3 0.... 20318us : __up_mutex_savestate (rt_run_flush)
    <...>-3 0.... 20318us : __down_mutex (rt_run_flush)
    <...>-3 0.... 20318us : __up_mutex_savestate (rt_run_flush)
    <...>-3 0.... 20319us : __down_mutex (rt_run_flush)


    "dmesg" says somethign like this:

    ( ubersock-4032 |#0): new 131 us user-latency.
    ( ubersock-4032 |#0): new 131 us user-latency.
    ( ubersock-4032 |#0): new 133 us user-latency.
    ( ubersock-4032 |#0): new 221 us user-latency.
    ( ubersock-4032 |#0): new 223 us user-latency.
    ( ubersock-4032 |#0): new 20629 us user-latency.

    When tracing I exit my test when a large latency is observed (in the
    case above a 20,629 usec value was observed by the "select()" test.

    If there is a more specific real-time newsgroup please direct me to it.

    G. Thaker

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