From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754388Ab0F3JDy (ORCPT ); Wed, 30 Jun 2010 05:03:54 -0400 Received: from vpn.id2.novell.com ([195.33.99.129]:56715 "EHLO vpn.id2.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752453Ab0F3JDx convert rfc822-to-8bit (ORCPT ); Wed, 30 Jun 2010 05:03:53 -0400 Message-Id: <4C2B24B30200007800008C07@vpn.id2.novell.com> X-Mailer: Novell GroupWise Internet Agent 8.0.1 Date: Wed, 30 Jun 2010 10:04:19 +0100 From: "Jan Beulich" To: "Peter Zijlstra" Cc: , , , "Ky Srinivasan" , , Subject: Re: [PATCH 2/4, v2] x86: enlightenment for ticket spin locks - Xen implementation References: <4C2A200F02000078000089E4@vpn.id2.novell.com> <1277885135.1868.72.camel@laptop> <4C2B21D20200007800008BDF@vpn.id2.novell.com> <1277888180.1868.84.camel@laptop> In-Reply-To: <1277888180.1868.84.camel@laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> On 30.06.10 at 10:56, Peter Zijlstra wrote: > On Wed, 2010-06-30 at 09:52 +0100, Jan Beulich wrote: >> > why is that a rwlock?, those things are useless. >> >> Because potentially each CPU's lock gets acquired for reading during >> unlock, while only the locking CPU's one needs to be acquired for >> writing during lock. > > Can you say: scalability nightmare? but then its Xen code so who cares.. Yes, this is a scalability problem. And yes, I'm open to suggestions. But no, I didn't have any better idea myself. And for the time being I don't foresee this to be a problem, as VMs tend to have much fewer CPUs than physical machines. And I think a performance win of 25% justifies a sub-optimal implementation (with the perspective of people smarter than me replacing it with a better one over time). Jan