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

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Message-Id: <200608291256.54665.ak@suse.de>
Date:	Tue, 29 Aug 2006 12:56:54 +0200
From:	Andi Kleen <ak@...e.de>
To:	David Howells <dhowells@...hat.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Dong Feng <middle.fengdong@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Christoph Lameter <clameter@....com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Why Semaphore Hardware-Dependent?

On Tuesday 29 August 2006 12:05, David Howells wrote:

>Because i386 (and x86_64) can do better by using XADDL/XADDQ.

x86-64 has always used the spinlock based version.

> On i386, CMPXCHG also ties you to what registers you may use for what to some
> extent. 

We've completely given up these kinds of micro optimization for spinlocks,
which are 1000x as critical as rwsems.  And nobody was able to benchmark
a difference.

It is very very likely nobody could benchmark a difference on rwsems either.

While I'm sure it's an interesting intellectual exercise to do these
advanced rwsems it would be better for everybody else to go for a single 
maintainable C implementation.

-Andi
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