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    SubjectRe: Scheduler benchmarks - a follow-up
    On 9/18/07, Ingo Molnar <mingo@elte.hu> wrote:
    >
    > * Rob Hussey <robjhussey@gmail.com> wrote:
    >
    > > The obligatory graphs:
    > > http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_lat_ctx_benchmark.png
    > > http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_hackbench_benchmark.png
    > > http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_pipe-test_benchmark.png
    >
    > btw., it's likely that if you turn off CONFIG_PREEMPT for .21 and for
    > .22-ck1 they'll improve a bit too - so it's not fair to put the .23
    > !PREEMPT numbers on the graph as the PREEMPT numbers of the other
    > kernels. (it shows the .23 scheduler being faster than it really is)
    >

    The graphs are really just to show where the new numbers fit in. Plus,
    I was too lazy to run all the numbers again.

    > > A cursory glance suggests that performance wrt lat_ctx and hackbench
    > > has increased (lower numbers), but degraded quite a lot for pipe-test.
    > > The numbers for pipe-test are extremely stable though, while the
    > > numbers for hackbench are more erratic (which isn't saying much since
    > > the original numbers gave nearly a straight line). I'm still willing
    > > to try out any more ideas.
    >
    > the pipe-test behavior looks like an outlier. !PREEMPT only removes code
    > (which makes the code faster), so this could be a cache layout artifact.
    > (or perhaps we preempt at a different point which is disadvantageous to
    > caching?) Pipe-test is equivalent to "lat_ctx -s 0 2" so if there was a
    > genuine slowdown it would show up in the lat_ctx graph - but the graph
    > shows a speedup.
    >

    Interestingly, every set of lat_ctx -s 0 2 numbers I run on the
    !PREEMPT kernel are on average higher than with PREEMPT (around 2.84
    for !PREEMPT and 2.4 for PREEMPT). Anything higher than around 2 or 3
    (such as lat_ctx -s 0 8) gives lower average numbers for !PREEMPT.

    Regards,
    Rob
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