linux-kernel - Re: [PATCH] Replace completions with semaphores

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Message-ID: <20080414154129.GB22259@elte.hu>
Date:	Mon, 14 Apr 2008 17:41:29 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>
Subject: Re: [PATCH] Replace completions with semaphores


* Matthew Wilcox <matthew@....cx> wrote:

> On Sun, Apr 13, 2008 at 09:05:13AM +0200, Ingo Molnar wrote:
> > there's also another aspect: completions are faster a bit 
> > in theory, because they know that they will schedule most of the time - 
> > while semaphores assume that they will _not_ schedule. (And that's 
> > exactly because the intent of the developer when using a completion is 
> > crystal clear.)
> 
> In practice though, the current implementation is slower. [...]

any URL to benchmarks?

> [...] Of course, that's fixable, and I strongly suspect that the 
> current users of completions simply don't care about speed -- the 
> normal use of completions is in startup and shutdown paths where a 
> millisecond extra isn't going to be noticable.

completions and semaphores act in the sub-microsecond range, not in the 
milliseconds range.

	Ingo
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