linux-kernel - Re: [PATCH 8/5] llist: Remove cpu

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Date:	Tue, 13 Sep 2011 14:43:39 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Huang Ying <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

On 09/12/2011 05:32 PM, Peter Zijlstra wrote:
> On Mon, 2011-09-12 at 17:26 +0300, Avi Kivity wrote:
> >  On 09/12/2011 05:05 PM, Peter Zijlstra wrote:
> >  >  Subject: llist: Remove cpu_relax() usage in cmpxchg loops
> >  >  From: Peter Zijlstra<a.p.zijlstra@...llo.nl>
> >  >  Date: Mon Sep 12 15:50:49 CEST 2011
> >  >
> >  >  Initial benchmarks show they're a net loss (2 socket wsm):
> >  >
> >  >    $ for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor ; do echo performance>   $i; done
> >  >    $ echo 4096 32000 64 128>   /proc/sys/kernel/sem
> >  >    $ ./sembench -t 2048 -w 1900 -o 0
> >  >
> >
> >  We hyperthreading enabled, and were all threads loaded?  cpu_relax
> >  allows the other thread to make more progress while the spinner relaxes.
>
> Yeah, with HT enabled, the benchmark runs 2048 tasks and does 1900 task
> bulk wakeups or so. Forgot the details, but it basically stresses the
> sleep+wakeup paths like nobodies business.

Ok.

Another issue is that hypervisors use PAUSE to detect a spinning guest 
and issue a directed yield to another vcpu.  But for cmpxchg loops, the 
"spinner" would just commit on the next loop, no?  So I think there's no 
objection from that front.

-- 
error compiling committee.c: too many arguments to function

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