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Subject: Re: 2.6.27-rc5 OLTP performance regression
From: Peter Zijlstra <peterz@infradead.org>
To: "Ma, Chinang" <chinang.ma@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>, Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
       Mike Galbraith <efault@gmx.de>, Gregory Haskins <ghaskins@novell.com>,
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       "Siddha, Suresh B" <suresh.b.siddha@intel.com>,
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Date: Mon, 08 Sep 2008 20:04:12 +0200
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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.