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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>,
       Steven Rostedt <rostedt@goodmis.org>,
       Nick Piggin <nickpiggin@yahoo.com.au>,
       "Siddha, Suresh B" <suresh.b.siddha@intel.com>,
       "Wilcox, Matthew R" <matthew.r.wilcox@intel.com>,
       "Tripathi, Sharad C" <sharad.c.tripathi@intel.com>,
       "Chilukuri, Harita" <harita.chilukuri@intel.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Date: Mon, 08 Sep 2008 20:00:33 +0200
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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.