linux-kernel - Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

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Message-ID: <20080311075845.GA13758@elte.hu>
Date:	Tue, 11 Mar 2008 08:58:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)


* Nick Piggin <nickpiggin@...oo.com.au> wrote:

> PostgreSQL is different. It has zero idle time when running this 
> workload. It actually scaled "super linearly" on my system here, from 
> single threaded performance to 8 cores (giving an 8.2x performance 
> increase)!
> 
> So PostgreSQL performance profile is actually much more interesting. 
> To my dismay, I found that Linux 2.6.25-rc5 performs really badly 
> after saturating the runqueues and subsequently increasing threads. 
> 2.6.22 drops a little bit, but basically settles near the peak 
> performance. With 2.6.25-rc5, throughput seems to be falling off 
> linearly with the number of threads.

thanks Nick, i'll check this - and i agree that this very much looks 
like a scheduler regression. Just a quick suggestion, does a simple 
runtime tune like this fix the workload:

  for N in /proc/sys/kernel/sched_domain/*/*/flags; do
     echo $[`cat $N`|16] > N
  done

this sets SD_WAKE_IDLE for all the nodes in the scheduler domains tree. 
(doing this results in over-agressive idle balancing - but if this fixes 
your testcase it shows that we were balancing under-agressively for this 
workload.) Thanks,

	Ingo
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