From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764096AbYDNRq5 (ORCPT ); Mon, 14 Apr 2008 13:46:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760703AbYDNRqt (ORCPT ); Mon, 14 Apr 2008 13:46:49 -0400 Received: from palinux.external.hp.com ([192.25.206.14]:46645 "EHLO mail.parisc-linux.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760788AbYDNRqt (ORCPT ); Mon, 14 Apr 2008 13:46:49 -0400 Date: Mon, 14 Apr 2008 11:46:32 -0600 From: Matthew Wilcox To: Ingo Molnar Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH] Replace completions with semaphores Message-ID: <20080414174631.GY11962@parisc-linux.org> References: <20080411210022.GJ11962@parisc-linux.org> <1208003081.7427.7.camel@twins> <20080413070513.GB19773@elte.hu> <20080413125205.GP11962@parisc-linux.org> <20080414154129.GB22259@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080414154129.GB22259@elte.hu> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 14, 2008 at 05:41:29PM +0200, Ingo Molnar wrote: > * Matthew Wilcox 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? No -- just reading the code. > > [...] 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. I've clearly misled both you and Jens, for which I apologise. My point was "even if they were a millisecond slower, nobody would notice", rather than "I've measured it and they're a millisecond slower" or even "from eyeballing it, I estimate they're about a millisecond slower". -- Intel are signing my paycheques ... these opinions are still mine "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step."