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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D31B6E81819 for ; Tue, 26 Sep 2023 04:10:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233467AbjIZEKH (ORCPT ); Tue, 26 Sep 2023 00:10:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47566 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229671AbjIZEKE (ORCPT ); Tue, 26 Sep 2023 00:10:04 -0400 Received: from 66-220-144-179.mail-mxout.facebook.com (66-220-144-179.mail-mxout.facebook.com [66.220.144.179]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F0450BF for ; Mon, 25 Sep 2023 21:09:57 -0700 (PDT) Received: by devbig1114.prn1.facebook.com (Postfix, from userid 425415) id 5E7A2C8F2415; Mon, 25 Sep 2023 21:09:42 -0700 (PDT) From: Stefan Roesch To: kernel-team@fb.com Cc: shr@devkernel.io, akpm@linux-foundation.org, david@redhat.com, hannes@cmpxchg.org, riel@surriel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v3 0/4] Smart scanning mode for KSM Date: Mon, 25 Sep 2023 21:09:35 -0700 Message-Id: <20230926040939.516161-1-shr@devkernel.io> X-Mailer: git-send-email 2.39.3 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This patch series adds "smart scanning" for KSM. What is smart scanning? =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D KSM evaluates all the candidate pages for each scan. It does not use hist= oric information from previous scans. This has the effect that candidate pages= that couldn't be used for KSM de-duplication continue to be evaluated for each= scan. The idea of "smart scanning" is to keep historic information. With the hi= storic information we can temporarily skip the candidate page for one or several= scans. Details: =3D=3D=3D=3D=3D=3D=3D=3D "Smart scanning" is to keep two small counters to store if the page has b= een used for KSM. One counter stores how often we already tried to use the pa= ge for KSM and the other counter stores how often we skip a page. How often we skip the candidate page depends how often a page failed KSM de-duplication. The code skips a maximum of 8 times. During testing this = has shown to be a good compromise for different workloads. New sysfs knob: =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Smart scanning is not enabled by default. With /sys/kernel/mm/ksm/smart_s= can smart scanning can be enabled. Monitoring: =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D To monitor how effective smart scanning is a new sysfs knob has been intr= oduced. /sys/kernel/mm/pages_skipped report how many pages have been skipped by s= mart scanning. Results: =3D=3D=3D=3D=3D=3D=3D=3D - Various workloads have shown a 20% - 25% reduction in page scans For the instagram workload for instance, the number of pages scanned ha= s been reduced from over 20M pages per scan to less than 15M pages. - Less pages scans also resulted in an overall higher de-duplication rate= as some shorter lived pages could be de-duplicated additionally - Less pages scanned allows to reduce the pages_to_scan parameter and this resulted in a 25% reduction in terms of CPU. - The improvements have been observed for workloads that enable KSM with madvise as well as prctl Changes: - V3: - Renamed field skip_age to remaining_skips - Moved fields after old_checksum - Changed should_skip_rmap_item to use remaining_skips field - V2: - Renamed function inc_skip_age() to skip_age() - Added comment to skip_age() function - Renamed function skip_rmap_item() to should_skip_rmap_item() - Added more comments to should_skip_rmap_item function - Added explicit modification of age with overflow check Stefan Roesch (4): mm/ksm: add "smart" page scanning mode mm/ksm: add pages_skipped metric mm/ksm: document smart scan mode mm/ksm: document pages_skipped sysfs knob Documentation/admin-guide/mm/ksm.rst | 11 +++ mm/ksm.c | 115 +++++++++++++++++++++++++++ 2 files changed, 126 insertions(+) base-commit: 15bcc9730fcd7526a3b92eff105d6701767a53bb --=20 2.39.3