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

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Message-ID: <809.1156876259@warthog.cambridge.redhat.com>
Date:	Tue, 29 Aug 2006 19:30:59 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Christoph Lameter <clameter@....com>,
	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? 

Andi Kleen <ak@...e.de> wrote:

> BTW maybe it would be a good idea to switch the wait list to a hlist,
> then the last user in the queue wouldn't need to 
> touch the cache line of the head. Or maybe even a single linked
> list then some more cache bounces might be avoidable.

You need a list_head to get O(1) push at one end and O(1) pop at the other.
In addition a singly-linked list makes interruptible ops non-O(1) also.

David
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