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    SubjectRe: [PATCH] Preemption Latency Measurement Tool
    Andre Pang wrote:
    >
    > On Sat, Sep 22, 2001 at 02:10:18AM -0400, Robert Love wrote:
    >
    > > > i did a test of it on linux-2.4.10-pre13 with Benno Senoner's
    > > > lowlatency program, which i hacked up a bit to output
    > > > /proc/latencytimes after each of the graphs. test results are at
    > > >
    > > > http://www.algorithm.com.au/hacking/linux-lowlatency/2.4.10-pre13-pes/
    > > >
    > > > and since i stared at the results in disbelief, i won't even try
    > > > to guess what's going on :). maybe you can make some sense of
    > > > it?
    > >
    > > Well, its not hard to decipher...and really, its actually fairly good.
    > > the latency test program is giving you a max latency of around 12ms in
    > > each test, which is OK.
    >
    > arrgh! i just realised my script buggered up and was producing the same
    > graph for all the results. please have a look at the page again, sorry.
    >
    > apart from that, i'm still confused. compared to other graphs produced
    > by the latencytest program, my system seems to have huge latencies.
    > unless i'm reading it wrongly, the graph is saying that i'm getting
    > latencies of up to 30ms, and a lot of overruns. compare this to
    >
    > http://www.gardena.net/benno/linux/audio/2.4.0-test2/3x256.html
    >
    > which shows latencytest on 2.4.0-test2, and
    >
    > http://www.gardena.net/benno/linux/audio/2.2.10-p133-3x128/3x128.html
    >
    > which are the results for latencytest on 2.2.10. admittedly these
    > kernels are much older, but i'm consistently getting far more latency
    > than those kernels. that's the bit i'm confused about :) i've tried
    > Andrew Morton's low-latency patches as well, to no avail. i've made
    > sure i've tuned my hard disks correctly, and i don't have any other
    > realtime processes running.
    >
    > am i concerned with a different issue than the one you're addressing?
    >
    > > the preemption-test patch is showing _MAX_ latencies of 0.8ms through
    > > 12ms. this is fine, too.
    >
    > yep, i agree with that ... so why is latencytest showing scheduling
    > latencies of > 30ms? i get the feeling i'm confusing two different
    > issues here. from what i understand, /proc/latencytimes shows the
    > how long it takes for various functions in the kernel to finish, and
    > the latencytest result shows how long it takes for it to be
    > re-scheduled (represented by the white line on the graph).

    The one thing the latancytimes patch doesn't monitor is interrupt off
    time. Maybe it should...


    George
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