linux-kernel - Re: [PATCH 0/4] pending scheduler updates

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Date:	Wed, 22 Oct 2008 14:10:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Galbraith <efault@....de>
Cc:	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] pending scheduler updates


* Mike Galbraith <efault@....de> wrote:

> On Wed, 2008-10-22 at 12:03 +0200, Mike Galbraith wrote:
> 
> > It has positive effects too, but IMHO, the bad outweigh the good.
> 
> BTW, most dramatic on the other end of the spectrum is pgsql+oltp.  
> With preemption as is, it collapses as load climbs to heavy with 
> preemption knobs at stock.  Postgres uses user-land spinlocks and 
> _appears_ to wake others while these are still held.  For this load, 
> there is such a thing as too much short-term fairness, preempting lock 
> holder creates nasty gaggle of contended lock spinners.  It's curable 
> with knobs, and I think it's postgres's own fault, but may be wrong.
> 
> With that patch, pgsql+oltp scales perfectly.

hm, tempting.

Have you tried to hack/fix pgsql to do proper wakeups?

Right now pgsql it punishes schedulers that preempt it while it is 
holding totally undeclared (to the kernel) user-space spinlocks ... 

Hence postgresql is rewarding a _bad_ scheduler policy in essence. And 
pgsql scalability seems to fall totally apart above 16 cpus - regardless 
of scheduler policy.

	Ingo
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