From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: [RFC v2 4/7] change kernel accounting to include steal time Date: Mon, 30 Aug 2010 12:07:51 -0700 Message-ID: <4C7C0187.7040401@goop.org> References: <1283184391-7785-1-git-send-email-glommer@redhat.com> <1283184391-7785-2-git-send-email-glommer@redhat.com> <1283184391-7785-3-git-send-email-glommer@redhat.com> <1283184391-7785-4-git-send-email-glommer@redhat.com> <1283184391-7785-5-git-send-email-glommer@redhat.com> <1283184391-7785-6-git-send-email-glommer@redhat.com> <1283184391-7785-7-git-send-email-glommer@redhat.com> <1283184391-7785-8-git-send-email-glommer@redhat.com> <4C7BEA9C.1060605@goop.org> <4C7BFACD.4030409@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Glauber Costa , kvm@vger.kernel.org, avi@redhat.com, zamsden@redhat.com, mtosatti@redhat.com, peterz@infradead.org, mingo@elte.hu To: Rik van Riel Return-path: Received: from claw.goop.org ([74.207.240.146]:55999 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751421Ab0H3THx (ORCPT ); Mon, 30 Aug 2010 15:07:53 -0400 In-Reply-To: <4C7BFACD.4030409@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 08/30/2010 11:39 AM, Rik van Riel wrote: > On 08/30/2010 01:30 PM, Jeremy Fitzhardinge wrote: >> On 08/30/2010 09:06 AM, Glauber Costa wrote: >>> This patch proposes a common steal time implementation. When no >>> steal time is accounted, we just add a branch to the current >>> accounting code, that shouldn't add much overhead. >> >> How is stolen time logically any different from a CPU running slowly due >> to HT or power management? Is it worth trying to handle them in the >> same way? (I'm mostly picking on the "_from_hypervisor" part, since >> that seems over-specific.) >> >> Why not have a get_unstolen_time() function which just returns >> sched_clock() in the normal case, but can return less? > > Steal time gets you information you can act on, when > your program is running slowly. > > The steal time statistic allows you to see whether the > slowdown was due to the CPU just not being fast enough, > or due to something else contending for the CPU. > > This can be useful information. Apparently, it has been > useful enough that it has been implemented on s390, PPC > and Xen (pre pvops Xen). Sure - pvops Xen has had stolen time accounting from the start (or a very long time, at least). I think Glauber's changes are functionally identical to Xen's existing stolen time support, but more intrusive. > I suppose HT can have similar results, but that is > also something that system administrators can see. My comments were mostly because I though that this patch was going to actually make the scheduler change behaviour to take stolen time into account in scheduling decisions - and those decisions would be equally meaningful if the slowdown was from HT, PM or stolen time. But AFAICT this patch does nothing to that end. J