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

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Message-Id: <200608292033.25194.ak@suse.de>
Date:	Tue, 29 Aug 2006 20:33:25 +0200
From:	Andi Kleen <ak@...e.de>
To:	David Howells <dhowells@...hat.com>
Cc:	Christoph Lameter <clameter@....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 Tuesday 29 August 2006 20:30, David Howells wrote:
> 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.

The poper should know its node address already because it's on its own stack.

> In addition a singly-linked list makes interruptible ops non-O(1) also.

When they are interrupted I guess? Hardly a problem to make that slower.

-Andi

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