Date: Wed, 22 Oct 2008 14:42:40 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Mike Galbraith <efault@gmx.de>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/4] pending scheduler updates
Message-ID: <20081022124240.GD25536@elte.hu>
References: <20081017172701.047939625@chello.nl> <20081020120538.GD10594@elte.hu> <20081021173529.GG11679@linux.vnet.ibm.com> <20081022094019.GG12453@elte.hu> <1224669821.6871.16.camel@marge.simson.net> <1224671528.7511.11.camel@marge.simson.net> <20081022121007.GG8095@elte.hu> <1224679082.12294.15.camel@marge.simson.net>
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* Mike Galbraith <efault@gmx.de> wrote:

> > > With that patch, pgsql+oltp scales perfectly.
> > 
> > hm, tempting.
> 
> I disagree.  Postgres's scaling problem is trivially corrected by 
> twiddling knobs (or whatnot). [...]

okay, then we need to document it a bit more: what knobs need twiddling 
to make it scale perfectly?

> [...]  With that patch, you can't twiddle mysql throughput back, or 
> disk intensive loads for that matter.  You can tweak the preempt 
> number, but it has nothing to do with lag, so anybody can preempt 
> anybody else as you turn the knob toward zero.  Chaos.

okay, convinced.

> > Have you tried to hack/fix pgsql to do proper wakeups?
> 
> No, I tried to build without spinlocks to verify, but build croaked. 
> Never went back to slogging through the code.

if it falls back to IPC semaphores that's a bad trade from a performance 
POV. The best would be if it used proper futexes (i.e. pthread_mutex() 
and friends) not some home-grown user-space spinlock thing.

	Ingo
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