From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7189EC43381 for ; Mon, 18 Feb 2019 17:45:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4A3532146F for ; Mon, 18 Feb 2019 17:45:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390613AbfBRRpM (ORCPT ); Mon, 18 Feb 2019 12:45:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37074 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731296AbfBRRpL (ORCPT ); Mon, 18 Feb 2019 12:45:11 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 27A7BC049E24; Mon, 18 Feb 2019 17:45:11 +0000 (UTC) Received: from sky.random (ovpn-120-13.rdu2.redhat.com [10.10.120.13]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B4777600C8; Mon, 18 Feb 2019 17:45:06 +0000 (UTC) Date: Mon, 18 Feb 2019 12:45:05 -0500 From: Andrea Arcangeli To: Jerome Glisse Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Peter Xu , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Andrew Morton , Matthew Wilcox , Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , Michal Hocko , kvm@vger.kernel.org Subject: Re: [RFC PATCH 0/4] Restore change_pte optimization to its former glory Message-ID: <20190218174505.GD30645@redhat.com> References: <20190131183706.20980-1-jglisse@redhat.com> <20190201235738.GA12463@redhat.com> <20190211190931.GA3908@redhat.com> <20190211200200.GA30128@redhat.com> <20190218160411.GA3142@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190218160411.GA3142@redhat.com> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 18 Feb 2019 17:45:11 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 18, 2019 at 11:04:13AM -0500, Jerome Glisse wrote: > So i run 2 exact same VMs side by side (copy of same COW image) and > built the same kernel tree inside each (that is the only important > workload that exist ;)) but the change_pte did not have any impact: > > before mean {real: 1358.250977, user: 16650.880859, sys: 839.199524, npages: 76855.390625} > before stdev {real: 6.744010, user: 108.863762, sys: 6.840437, npages: 1868.071899} > after mean {real: 1357.833740, user: 16685.849609, sys: 839.646973, npages: 76210.601562} > after stdev {real: 5.124797, user: 78.469360, sys: 7.009164, npages: 2468.017578} > without mean {real: 1358.501343, user: 16674.478516, sys: 837.791992, npages: 76225.203125} > without stdev {real: 5.541104, user: 97.998367, sys: 6.715869, npages: 1682.392578} > > Above is time taken by make inside each VM for all yes config. npages > is the number of page shared reported on the host at the end of the > build. Did you set /sys/kernel/mm/ksm/sleep_millisecs to 0? It would also help to remove the checksum check from mm/ksm.c: - if (rmap_item->oldchecksum != checksum) { - rmap_item->oldchecksum = checksum; - return; - } One way or another, /sys/kernel/mm/ksm/pages_shared and/or pages_sharing need to change significantly to be sure we're exercising the COW/merging code that uses change_pte. KSM is smart enough to merge only not frequently changing pages, and with the default KSM code this probably works too well for a kernel build. > Should we still restore change_pte() ? It does not hurt, but it does > not seems to help in anyway. Maybe you have a better benchmark i could > run ? We could also try a microbenchmark based on ltp/testcases/kernel/mem/ksm/ksm02.c that already should trigger a merge flood and a COW flood during its internal processing. Thanks, Andrea