From owner-freebsd-current@FreeBSD.ORG Sun Feb 25 10:56:13 2007 Return-Path: X-Original-To: freebsd-current@freebsd.org Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 3844616A402 for ; Sun, 25 Feb 2007 10:56:13 +0000 (UTC) (envelope-from rwatson@FreeBSD.org) Received: from cyrus.watson.org (cyrus.watson.org [209.31.154.42]) by mx1.freebsd.org (Postfix) with ESMTP id 0E9D313C4A6 for ; Sun, 25 Feb 2007 10:56:12 +0000 (UTC) (envelope-from rwatson@FreeBSD.org) Received: from fledge.watson.org (fledge.watson.org [209.31.154.41]) by cyrus.watson.org (Postfix) with ESMTP id AF925485C4; Sun, 25 Feb 2007 05:56:12 -0500 (EST) Date: Sun, 25 Feb 2007 10:56:12 +0000 (GMT) From: Robert Watson X-X-Sender: robert@fledge.watson.org To: Ivan Voras In-Reply-To: Message-ID: <20070225105220.N36322@fledge.watson.org> References: <20070224213111.GB41434@xor.obsecurity.org> <346a80220702242100i7ec22b5h4b25cc7d20d03e98@mail.gmail.com> <20070225054120.GA47059@xor.obsecurity.org> <1C143520-B893-4F43-8F7E-04B021D2EE69@siliconlandmark.com> <20070225060908.GA47476@xor.obsecurity.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: freebsd-current@freebsd.org Subject: Re: Progress on scaling of FreeBSD on 8 CPU systems X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 25 Feb 2007 10:56:13 -0000 On Sun, 25 Feb 2007, Ivan Voras wrote: > Kris Kennaway wrote: > >> Hopefully within a week or two. It might not be that exact patch, I >> think John wants to try and do it a bit differently instead of >> introducing a new locking primitive just for this. > > Well why not? :) I am not an expert, but reading jeffr's posts it looks like > the idea of sleepable mutexes was taken from Solaris, where it's also not > exactly documented. If moving away from sleepable mutexes introduces more > than a small single digit percentage drop in performance (1% on > multi-gigahertz machines is a lot), why not keep it? If it's dangerous to > use, that should be documented in the man page with big bold letters but if > it helps, keep it. > > (Of course I might be completely off the track and sleepable mutexes might > be inconsequential for performance here :) ) Well, there are two ways you can ask the question about locks here: (1) Why don't we allow sleeping with mutexes? (2) Why don't the sleepable locking primitives perform better? There are now patches that address this from both sides, optimizing sx lock performance and allowing mutexes to sleep. There are serious deadlock issues that can arise with sleepable mutexes; I believe Jeff's patch includes the necessary bits to teach WITNESS how to detect some misuse at run-time. Right now, with the exception of the fast interrupt context, mutexes are universally acquirable in any context subject to lock order. If we have sleepable mutexes, this will no longer be true, which is a significant change in the constraints on use. Attilio has a heavily optimized sxlock implementation as well, although I'm not sure the two have been benchmarked side-by-side, but that would be an obvious next thing to try. Robert N M Watson Computer Laboratory University of Cambridge