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 41005E7C4E9 for ; Wed, 4 Oct 2023 19:07:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245478AbjJDTHx (ORCPT ); Wed, 4 Oct 2023 15:07:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40060 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1343887AbjJDTHY (ORCPT ); Wed, 4 Oct 2023 15:07:24 -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 DF76630EC for ; Wed, 4 Oct 2023 12:03:15 -0700 (PDT) Received: by devbig1114.prn1.facebook.com (Postfix, from userid 425415) id 5BCF4D08BFF9; Wed, 4 Oct 2023 12:03:02 -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 v1 0/4] mm/ksm: Add ksm advisor Date: Wed, 4 Oct 2023 12:02:45 -0700 Message-Id: <20231004190249.829015-1-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 What is the KSM advisor? =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D The ksm advisor automatically manages the pages_to_scan setting to achieve a target scan time. The target scan time defines how many seconds it should take to scan all the candidate KSM pages. In other words the pages_to_scan rate is changed by the advisor to achieve the target scan time. Why do we need a KSM advisor? =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D The number of candidate pages for KSM is dynamic. It can often be observe= d that during the startup of an application more candidate pages need to be processed. Without an advisor the pages_to_scan parameter needs to be sized for the maximum number of candidate pages. With the scan time advisor the pages_to_scan parameter based can be changed based on demand. Algorithm =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D The algorithm calculates the change value based on the target scan time and the previous scan time. To avoid pertubations an exponentially weighted moving average is applied. The algorithm has a max and min value to: - guarantee responsiveness to changes - to avoid to spend too much CPU Parameters to influence the KSM scan advisor =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D The respective parameters are: - ksm_advisor_mode 0: None (default), 1: scan time advisor - ksm_advisor_target_scan_time how many seconds a scan should of all candidate pages take - ksm_advisor_min_pages minimum value for pages_to_scan per batch - ksm_advisor_max_pages maximum value for pages_to_scan per batch The parameters are exposed as knobs in /sys/kernel/mm/ksm. By default the scan time advisor is disabled. Currently there are two advisors: - none and - scan time. Resource savings =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Tests with various workloads have shown considerable CPU savings. Most of the workloads I have investigated have more candidate pages during startup, once the workload is stable in terms of memory, the number of candidate pages is reduced. Without the advisor, the pages_to_scan needs to be sized for the maximum number of candidate pages. So having this advisor definitely helps in reducing CPU consumption. For the instagram workload, the advisor achieves a 25% CPU reduction. Once the memory is stable, the pages_to_scan parameter gets reduced to about 40% of its max value. The new advisor works especially well if the smart scan feature is also enabled. How is defining a target scan time better? =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D For an administrator it is more logical to set a target scan time.. The administrator can determine how many pages are scanned on each scan. Therefore setting a target scan time makes more sense. In addition the administrator might have a good idea about the memory sizing of its respective workloads. The pages_to_scan parameter is per batch. But the administrator has no insight how many batches are needed per scan. Tracing =3D=3D=3D=3D=3D=3D=3D A new tracing event has been added for the scan time advisor. The new trace event is called ksm_advisor. It reports the scan time and the new pages_to_scan setting. Other approaches =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Approach 1: Adapt pages_to_scan after processing each batch. If KSM merges pages, increase the scan rate, if less KSM pages, reduce the the pages_to_scan rate. This doesn't work too well. While it increases the pages_to_scan for a short period, but generally it ends up with a too low pages_to_scan rate. Approach 2: Adapt pages_to_scan after each scan. The problem with that approach is that the calculated scan rate tends to be high. The more aggressive KSM scans, the more pages it can de-duplicate. There have been earlier attempts at an advisor: propose auto-run mode of ksm and its tests (https://marc.info/?l=3Dlinux-mm&m=3D166029880214485&w=3D2) Stefan Roesch (4): mm/ksm: add ksm advisor mm/ksm: add sysfs knobs for advisor mm/ksm: add tracepoint for ksm advisor mm/ksm: document ksm advisor and its sysfs knobs Documentation/admin-guide/mm/ksm.rst | 45 +++++ include/trace/events/ksm.h | 28 ++++ mm/ksm.c | 242 ++++++++++++++++++++++++++- 3 files changed, 314 insertions(+), 1 deletion(-) base-commit: 12d04a7bf0da67321229d2bc8b1a7074d65415a9 --=20 2.39.3