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Subject: Re: CFS scheduler OLTP perforamnce
From: Peter Zijlstra <peterz@infradead.org>
To: "Ma, Chinang" <chinang.ma@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "Wilcox, Matthew R" <matthew.r.wilcox@intel.com>,
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       "Styner, Douglas W" <douglas.w.styner@intel.com>,
       "Chilukuri, Harita" <harita.chilukuri@intel.com>,
       "Wang, Peter Xihong" <peter.xihong.wang@intel.com>,
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Date: Fri, 12 Dec 2008 14:38:20 +0100
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On Fri, 2008-12-12 at 13:12 +0100, Peter Zijlstra wrote:
> On Thu, 2008-12-11 at 16:25 -0700, Ma, Chinang wrote:
> > We are evaluating the CFS OLTP performance with 2.6.28-c7 kernel. In
> > this workload once a database foreground process commit a transaction
> > it will signal the log writer process to write to the log file.
> > Foreground processes will wait until log writer finish writing and
> > wake them up. With hundreds of foreground process running in the
> > system, it is important that the log writer get to run as soon as data
> > is available. 
> > 
> > Here are the experiments we have done with 2.6.28-rc7.
> > 1. Increase log writer priority "renice -20 <log writer pid>" while
> > keeping all other processes running in default CFS priority. We get a
> > baseline performance with log latency (scheduling + i/o) at 7 ms.
> 
> Is this better or the same than nice-0 ?
> 
> > 2. To reduce log latency, we set log writer to SCHED_RR with higher
> > priority. We tried "chrt -p 49  <log writer pid>" and got 0.7% boost
> > in performance with log latency reduced to 6.4 ms.

BTW, 6.4ms schedule latency sounds insanely long for a RR task, are you
running a PREEMPT=n kernel or something?

How would you characterize the log tasks behaviour?

 - does it run long/short (any quantization)
 - does it sleep long/short - how does it compare to its runtime?
 - does it wake others?
   - if so, always the one who woke it, or multiple others?


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