linux-kernel - Re: Significant performance difference for postgres w/o sched

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Message-ID: <1436409938.3477.31.camel@gmx.de>
Date:	Thu, 09 Jul 2015 04:45:38 +0200
From:	Mike Galbraith <efault@....de>
To:	Andres Freund <andres@...razel.de>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Significant performance difference for postgres w/o
 sched_autogroup

On Wed, 2015-07-08 at 17:45 +0200, Andres Freund wrote:

> Workload:
> 
> postgresql (multi-process via shared memory SQL server) and benchmark
> client (pgbench, multi threaded) running on the same server. Connected
> using unix sockets.  The statements are relatively simple (~1.5ms on
> average), forcing frequent back/forth between server processes and
> pgbench threads.
> 
> I found that disabling sched_autogroup *significantly* reduces
> throughput. Even when both server and client are started from the same
> terminal and thus should be in the same group!
> 
> There's a significant difference in how %sys with autogroups
> enabled/disabled. ~8% v ~27%. That sounds too much.

Seems reasonable to me.  1(tg)/2(tgs) > 1(task)/N(tasks), throughput is
what the server can sustain on its given budget, larger budget means
less client blockage, thus less %sys.

	-Mike

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