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    SubjectRe: 2.6.27-rc5 OLTP performance regression
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    On Mon, 2008-09-08 at 20:00 +0200, Peter Zijlstra wrote:
    > On Mon, 2008-09-08 at 10:58 -0700, Ma, Chinang wrote:
    > > We found the group scheduler in 2.6.27-rc5 has negative performance
    > > impact on TPC Online Transaction Processing workload. The test was
    > > conducted on a dual-socket quad-core Xeon server. OLTP workload is
    > > disk i/o intensive and we have over 200 database shadow processes
    > > running in the server during this test. Enabling group scheduler
    > > (CONFIG_GROUP_SCHED=y) reduced performance by 2.0%. Oprofile data
    > > indicates significant amount of cycles are spent in tg_shares_up().
    > > This is new regression as we did not find the same issue with 2.6.26
    > > kernel group scheduler. Is anybody looking into group scheduler
    > > performance and any idea for reducing the performance impact?
    >
    > Because the .26 group scheduler wasn't SMP aware. The extra cost comes
    > from the fact that .27 is.

    What you can do it increase the /proc/sys/kernel/sched_shares_ratelimit
    value and thereby decrease the accuracy of the SMP fairness of the group
    scheduler.



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