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

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Date:	Tue, 11 Mar 2008 19:28:19 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

On Tuesday 11 March 2008 18:58, Ingo Molnar wrote:
> * 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

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.

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