From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965502AbXCMJZW (ORCPT ); Tue, 13 Mar 2007 05:25:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S965607AbXCMJZW (ORCPT ); Tue, 13 Mar 2007 05:25:22 -0400 Received: from mailhub.sw.ru ([195.214.233.200]:44614 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965502AbXCMJZV (ORCPT ); Tue, 13 Mar 2007 05:25:21 -0400 Message-ID: <45F66E73.1010006@sw.ru> Date: Tue, 13 Mar 2007 12:27:15 +0300 From: Pavel Emelianov User-Agent: Thunderbird 1.5 (X11/20060317) MIME-Version: 1.0 To: "Eric W. Biederman" CC: Herbert Poetzl , containers@lists.osdl.org, Paul Menage , Linux Kernel Mailing List Subject: Re: [RFC][PATCH 1/7] Resource counters References: <45ED7DEC.7010403@sw.ru> <45ED7F69.60108@sw.ru> <45EE39A5.7010804@in.ibm.com> <45EE6769.9060701@sw.ru> <20070309163711.GA3647@MAIL.13thfloor.at> <20070312011612.GD21861@MAIL.13thfloor.at> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Eric W. Biederman wrote: > Herbert Poetzl writes: > >> On Sun, Mar 11, 2007 at 01:00:15PM -0600, Eric W. Biederman wrote: >>> Herbert Poetzl writes: >>> >>>> Linux-VServer does the accounting with atomic counters, >>>> so that works quite fine, just do the checks at the >>>> beginning of whatever resource allocation and the >>>> accounting once the resource is acquired ... >>> Atomic operations versus locks is only a granularity thing. >>> You still need the cache line which is the cost on SMP. >>> >>> Are you using atomic_add_return or atomic_add_unless or >>> are you performing you actions in two separate steps >>> which is racy? What I have seen indicates you are using >>> a racy two separate operation form. >> yes, this is the current implementation which >> is more than sufficient, but I'm aware of the >> potential issues here, and I have an experimental >> patch sitting here which removes this race with >> the following change: >> >> - doesn't store the accounted value but >> limit - accounted (i.e. the free resource) >> - uses atomic_add_return() >> - when negative, an error is returned and >> the resource amount is added back >> >> changes to the limit have to adjust the 'current' >> value too, but that is again simple and atomic >> >> best, >> Herbert >> >> PS: atomic_add_unless() didn't exist back then >> (at least I think so) but that might be an option >> too ... > > I think as far as having this discussion if you can remove that race > people will be more willing to talk about what vserver does. > > That said anything that uses locks or atomic operations (finer grained locks) > because of the cache line ping pong is going to have scaling issues on large > boxes. BTW atomic_add_unless() is essentially a loop!!! Just like spin_lock() is, so why is one better that another? spin_lock() can go to schedule() on preemptive kernels thus increasing interactivity, while atomic can't. > So in that sense anything short of per cpu variables sucks at scale. That said > I would much rather get a simple correct version without the complexity of > per cpu counters, before we optimize the counters that much. > > Eric >