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Subjectmore on scheduler benchmarks
Last week while discussing scheduler benchmarks, Bill Hartner
made a comment something like the following "the benchmark may
not even be invoking the scheduler as you expect". This comment
did not fully sink in until this weekend when I started thinking
about changes made to sched_yield() in 2.4.0. (I'm cc'ing Ingo
Molnar because I think he was involved in the changes). If you
haven't taken a look at sys_sched_yield() in 2.4.0, I suggest
that you do that now.

A result of new optimizations made to sys_sched_yield() is that
calling sched_yield() does not result in a 'reschedule' if there
are no tasks waiting for CPU resources. Therefore, I would claim
that running 'scheduler benchmarks' which loop doing sched_yield()
seem to have little meaning/value for runs where the number of
looping tasks is less than then number of CPUs in the system. Is
that an accurate statement?

If the above is accurate, then I am wondering what would be a
good scheduler benchmark for these low task count situations.
I could undo the optimizations in sys_sched_yield() (for testing
purposes only!), and run the existing benchmarks. Can anyone
suggest a better solution?

Mike Kravetz
IBM Linux Technology Center
15450 SW Koll Parkway
Beaverton, OR 97006-6063 (503)578-3494
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