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[209.132.183.28]) by mx.google.com with ESMTPS id 6si4720904qvm.65.2019.02.18.09.45.11 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 18 Feb 2019 09:45:12 -0800 (PST) Received-SPF: pass (google.com: domain of aarcange@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of aarcange@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=aarcange@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com 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) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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