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    SubjectRe: Scheduler benchmarks - a follow-up

    * Rob Hussey <robjhussey@gmail.com> wrote:

    > Hi all,
    >
    > After posting some benchmarks involving cfs
    > (http://lkml.org/lkml/2007/9/13/385), I got some feedback, so I
    > decided to do a follow-up that'll hopefully fill in the gaps many
    > people wanted to see filled.

    thanks for the update!

    > I'll start with some selected numbers, which are preceded by the
    > command used for the benchmark.
    >
    > for((i=2; i < 201; i++)); do lat_ctx -s 0 $i; done:
    > (the left most column is the number of processes ($i))
    >
    > 2.6.21 2.6.22-ck1 2.6.23-rc6-cfs-devel
    >
    > 15 5.88 4.85 5.14
    > 16 5.80 4.77 4.76

    the unbound results are harder to compare because CFS changed SMP
    balancing to saturate multiple cores better - but this can result in a
    micro-benchmark slowdown if the other core is idle (and one of the
    benchmark tasks runs on one core and the other runs on the first core).
    This affects lat_ctx and pipe-test. (I'll have a look at the hackbench
    behavior.)

    > Bound to Single core:

    these are the more comparable (apples to apples) tests. Usually the most
    stable of them is pipe-test:

    > pipe-test:
    >
    > 2.6.21 2.6.22-ck1 2.6.23-rc6-cfs-devel
    >
    > 1 9.27 8.50 8.55
    > 2 9.27 8.47 8.55
    > 3 9.28 8.47 8.54
    > 4 9.28 8.48 8.54
    > 5 9.28 8.48 8.54

    so -ck1 is 0.8% faster in this particular test. (but still, there can be
    caching effects in either direction - so i usually run the test on both
    cores/CPUs to see whether there's any systematic spread in the results.
    The cache-layout related random spread can be as high as 10% on some
    systems!)

    many things happened between 2.6.22-ck1 and 2.6.23-cfs-devel that could
    affect performance of this test. My initial guess would be sched_clock()
    overhead. Could you send me your system's 'dmesg' output when running a
    2.6.22 (or -ck1) kernel? Chances are that your TSC got marked unstable,
    this turns on a much less precise but also faster sched_clock()
    implementation. CFS uses the TSC even if the time-of-day code marked it
    as unstable - going for the more precise but slightly slower variant.

    To test this theory, could you apply the patch below to cfs-devel (if
    you are interested in further testing this) - this changes the cfs-devel
    version of sched_clock() to have a low-resolution fallback like v2.6.22
    does. Does this result in any measurable increase in performance?

    (there's also a new sched-devel.git tree out there - if you update to it
    you'll need to re-pull it against a pristine Linus git head.)

    Ingo

    ---
    arch/i386/kernel/tsc.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    Index: linux/arch/i386/kernel/tsc.c
    ===================================================================
    --- linux.orig/arch/i386/kernel/tsc.c
    +++ linux/arch/i386/kernel/tsc.c
    @@ -110,9 +110,9 @@ unsigned long long native_sched_clock(vo
    * very important for it to be as fast as the platform
    * can achive it. )
    */
    - if (unlikely(!tsc_enabled && !tsc_unstable))
    + if (1 || unlikely(!tsc_enabled && !tsc_unstable))
    /* No locking but a rare wrong value is not a big deal: */
    - return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
    + return jiffies_64 * (1000000000 / HZ);

    /* read the Time Stamp Counter: */
    rdtscll(this_offset);
    -
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