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    SubjectRe: O(1) scheduler gives big boost to tbench 192
    On Wed, May 08, 2002 at 09:31:39AM -0700, Robert Love wrote:
    > On Wed, 2002-05-08 at 08:34, Jussi Laako wrote:
    > >
    > > Maybe this is the reason why O(1) scheduler has big latencies with
    > > pthread_cond_*() functions which original scheduler doesn't have?
    > > I think I tracked the problem down to try_to_wake_up(), but I was unable to
    > > fix it.
    > Ah this could be the same case. I just looked into the definition of
    > the conditional variable pthread stuff and it looks like it _could_ be
    > implemented using pipes but I do not see why it would per se. If it
    > does not use pipes, then this sync issue is not at hand (only the pipe
    > code passed 1 for the sync flag).
    > If it does not use pipes, we could have another problem - but I doubt
    > it. Maybe the benchmark is just another case where it shows worse
    > performance due to some attribute of the scheduler or load balancer?

    In some cases, the O(1) scheduler will produce higher latencies than
    the old scheduler. On 'some' workloads/benchmarks the old scheduler
    was better because it had a greater tendency to schedule tasks on the
    same CPU. This is certainly the case with the lat_ctx and lat_pipe
    components of LMbench. Note that this has nothing to do with the
    wake_up sync behavior. Rather, it is the difference between scheduling
    a new task on the current CPU as opposed to a 'remote' CPU. You can
    schedule the task on the current CPU quicker, but this is not good for
    optimal cache usage. I believe the O(1) scheduler makes the correct
    trade off in this area.

    Is there anything simple I can do to check the latencies of the
    pthread_cond_*() functions? I'd like to do some analysis of
    scheduler behavior, but am unfamiliar with the user level code.

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