RE: 2.6.27-rc5 OLTP performance regression

[Ma, Chinang]

Peter,
        We increased sched_shares_ratelimit and were able to recover the 2% performance regression.
Thanks,

-----Original Message-----
From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
Sent: Monday, September 08, 2008 11:04 AM
To: Ma, Chinang
Cc: Ingo Molnar; Srivatsa Vaddagiri; Mike Galbraith; Gregory Haskins; Steven Rostedt; Nick Piggin; Siddha, Suresh B; Wilcox, Matthew R; Tripathi, Sharad C; Chilukuri, Harita; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: 2.6.27-rc5 OLTP performance regression

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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