From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031783Ab2COSCF (ORCPT ); Thu, 15 Mar 2012 14:02:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42400 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752818Ab2COSCD (ORCPT ); Thu, 15 Mar 2012 14:02:03 -0400 Message-ID: <4F622E90.5080001@redhat.com> Date: Thu, 15 Mar 2012 20:01:52 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120216 Thunderbird/10.0.1 MIME-Version: 1.0 To: Dan Magenheimer CC: Akshay Karle , linux-kernel@vger.kernel.org, Konrad Wilk , kvm@vger.kernel.org, ashu tripathi , nishant gulhane , amarmore2006 , Shreyas Mahure , mahesh mohan Subject: Re: [RFC 0/2] kvm: Transcendent Memory (tmem) on KVM References: <1331224181.2585.16.camel@aks> <4F621FC0.7050800@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/15/2012 07:49 PM, Dan Magenheimer wrote: > > One of the potential problems with tmem is reduction in performance when > > the cache hit rate is low, for example when streaming. > > > > Can you test this by creating a large file, for example with > > > > dd < /dev/urandom > file bs=1M count=100000 > > > > and then measuring the time to stream it, using > > > > time dd < file > /dev/null > > > > with and without the patch? > > > > Should be done on a cleancache enabled guest filesystem backed by a > > virtio disk with cache=none. > > > > It would be interesting to compare kvm_stat during the streaming, with > > and without the patch. > > Hi Avi -- > > The "WasActive" patch (https://lkml.org/lkml/2012/1/25/300) > is intended to avoid the streaming situation you are creating here. > It increases the "quality" of cached pages placed into zcache > and should probably also be used on the guest-side stubs (and/or maybe > the host-side zcache... I don't know KVM well enough to determine > if that would work). > > As Dave Hansen pointed out, the WasActive patch is not yet correct > and, as akpm points out, pageflag bits are scarce on 32-bit systems, > so it remains to be seen if the WasActive patch can be upstreamed. > Or maybe there is a different way to achieve the same goal. > But I wanted to let you know that the streaming issue is understood > and needs to be resolved for some cleancache backends just as it was > resolved in the core mm code. Nice. This takes care of the tail-end of the streaming (the more important one - since it always involves a cold copy). What about the other side? Won't the read code invoke cleancache_get_page() for every page? (this one is just a null hypercall, so it's cheaper, but still expensive). > The measurement you suggest would still be interesting even > without the WasActive patch as it measures a "worst case". It can provide the justification for that patch, yes. -- error compiling committee.c: too many arguments to function