linux-kernel - Re: Why Semaphore Hardware-Dependent?

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Message-ID: <Pine.LNX.4.64.0608291033380.19174@schroedinger.engr.sgi.com>
Date:	Tue, 29 Aug 2006 10:36:16 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andi Kleen <ak@...e.de>
cc:	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Dong Feng <middle.fengdong@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Why Semaphore Hardware-Dependent?

On Tue, 29 Aug 2006, Andi Kleen wrote:

> On Tuesday 29 August 2006 17:56, Christoph Lameter wrote:
> > On Tue, 29 Aug 2006, David Howells wrote:
> > 
> > > Because i386 (and x86_64) can do better by using XADDL/XADDQ.
> > 
> > And Ia64 would like to use fetchadd....
> 
> This might be a dumb question, but I would expect even on altix 
> with lots of parallel faulting threads rwsem performance be basically
> limited by aquiring the cache line and releasing it later to another CPU.

Correct. However, a cmpxchg may have to acquire that cacheline multiple 
times in a highly contented situation. A fetchadd acquires the cacheline 
only once.

> Do you really think it will make much difference what particular atomic
> operation is used? The basic cost of sending the cache line over the
> interconnect should be all the same, no? And once the cache line is local
> it should be reasonably fast either way.

We have long tuned that portion of the code and therefore we are 
skeptical of changes. But if we cannot measure a difference to a 
generic implemenentation then it would be okay.

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