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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Ingo Molnar <mingo@elte.hu>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)
Date: Tue, 11 Mar 2008 19:28:19 +1100	[thread overview]
Message-ID: <200803111928.20043.nickpiggin@yahoo.com.au> (raw)
In-Reply-To: <20080311075845.GA13758@elte.hu>

On Tuesday 11 March 2008 18:58, Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@yahoo.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

Thanks.


> - and i agree that this very much looks 
> like a scheduler regression.

I'd say it is. Quite a nasty one too: if your server gets nudged over
the edge of the cliff, it goes into a feedback loop and goes splat at
the bottom somewhere ;)


> 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,

It doesn't change anything.

There is no idle time for this workload, btw.


  reply	other threads:[~2008-03-11  8:29 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-11  6:49 Nick Piggin
2008-03-11  7:58 ` Ingo Molnar
2008-03-11  8:28   ` Nick Piggin [this message]
2008-03-11  9:59     ` Ingo Molnar
2008-03-11 21:07 ` Nicholas Miell
2008-03-11 21:34   ` Cyrus Massoumi
2008-03-11 23:12     ` Nicholas Miell
2008-03-11 23:42       ` Nick Piggin
2008-03-11 23:47       ` Stephen Hemminger
2008-03-12  9:00       ` Zhang, Yanmin
     [not found] ` <20080311102538.GA30551@elte.hu>
     [not found]   ` <20080311120230.GA5386@elte.hu>
2008-03-12  1:21     ` Nick Piggin
2008-03-12  7:58       ` Peter Zijlstra
2008-03-17  0:44         ` Nick Piggin
2008-03-17  5:16           ` Ray Lee
2008-03-17  5:21             ` Willy Tarreau
2008-03-17  7:19               ` Nick Piggin
2008-03-17  8:26                 ` Willy Tarreau
2008-03-17  8:54                   ` Nick Piggin
2008-03-17  9:28                     ` Peter Zijlstra
2008-03-17  9:56                       ` Nick Piggin
2008-03-17 10:16                       ` Ingo Molnar
2008-03-17  5:34             ` Nick Piggin
2008-03-14 15:42 Xose Vazquez Perez

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