From nobody Sun Dec 28 02:50:05 2025 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 DE35EC4167D for ; Wed, 13 Dec 2023 18:28:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1442625AbjLMS1y (ORCPT ); Wed, 13 Dec 2023 13:27:54 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58676 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235536AbjLMS1k (ORCPT ); Wed, 13 Dec 2023 13:27:40 -0500 Received: from 66-220-144-178.mail-mxout.facebook.com (66-220-144-178.mail-mxout.facebook.com [66.220.144.178]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E6FDB11B for ; Wed, 13 Dec 2023 10:27:46 -0800 (PST) Received: by devbig1114.prn1.facebook.com (Postfix, from userid 425415) id C6E0610CFA72F; Wed, 13 Dec 2023 10:27:33 -0800 (PST) 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 v4 4/4] mm/ksm: document ksm advisor and its sysfs knobs Date: Wed, 13 Dec 2023 10:27:29 -0800 Message-Id: <20231213182729.587081-5-shr@devkernel.io> X-Mailer: git-send-email 2.39.3 In-Reply-To: <20231213182729.587081-1-shr@devkernel.io> References: <20231213182729.587081-1-shr@devkernel.io> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" This documents the KSM advisor and its new knobs in /sys/fs/kernel/mm. Signed-off-by: Stefan Roesch Acked-by: David Hildenbrand --- Documentation/admin-guide/mm/ksm.rst | 55 ++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-gui= de/mm/ksm.rst index e59231ac6bb71..a639cac124777 100644 --- a/Documentation/admin-guide/mm/ksm.rst +++ b/Documentation/admin-guide/mm/ksm.rst @@ -80,6 +80,9 @@ pages_to_scan how many pages to scan before ksmd goes to sleep e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``. =20 + The pages_to_scan value cannot be changed if ``advisor_mode`` has + been set to scan-time. + Default: 100 (chosen for demonstration purposes) =20 sleep_millisecs @@ -164,6 +167,29 @@ smart_scan optimization is enabled. The ``pages_skipped`` metric shows how effective the setting is. =20 +advisor_mode + The ``advisor_mode`` selects the current advisor. Two modes are + supported: none and scan-time. The default is none. By setting + ``advisor_mode`` to scan-time, the scan time advisor is enabled. + The section about ``advisor`` explains in detail how the scan time + advisor works. + +adivsor_max_cpu + specifies the upper limit of the cpu percent usage of the ksmd + background thread. The default is 70. + +advisor_target_scan_time + specifies the target scan time in seconds to scan all the candidate + pages. The default value is 200 seconds. + +advisor_min_pages_to_scan + specifies the lower limit of the ``pages_to_scan`` parameter of the + scan time advisor. The default is 500. + +adivsor_max_pages_to_scan + specifies the upper limit of the ``pages_to_scan`` parameter of the + scan time advisor. The default is 30000. + The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/k= sm/``: =20 general_profit @@ -263,6 +289,35 @@ ksm_swpin_copy note that KSM page might be copied when swapping in because do_swap_page() cannot do all the locking needed to reconstitute a cross-anon_vma KSM pag= e. =20 +Advisor +=3D=3D=3D=3D=3D=3D=3D + +The number of candidate pages for KSM is dynamic. It can be often observed +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. The scan time advisor can +changes the ``pages_to_scan`` parameter based on demand. + +The advisor can be enabled, so KSM can automatically adapt to changes in t= he +number of candidate pages to scan. Two advisors are implemented: none and +scan-time. With none, no advisor is enabled. The default is none. + +The scan time advisor changes the ``pages_to_scan`` parameter based on the +observed scan times. The possible values for the ``pages_to_scan`` paramet= er is +limited by the ``advisor_max_cpu`` parameter. In addition there is also the +``advisor_target_scan_time`` parameter. This parameter sets the target tim= e to +scan all the KSM candidate pages. The parameter ``advisor_target_scan_time= `` +decides how aggressive the scan time advisor scans candidate pages. Lower +values make the scan time advisor to scan more aggresively. This is the mo= st +important parameter for the configuration of the scan time advisor. + +The initial value and the maximum value can be changed with +``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The defau= lt +values are sufficient for most workloads and use cases. + +The ``pages_to_scan`` parameter is re-calculated after a scan has been com= pleted. + + -- Izik Eidus, Hugh Dickins, 17 Nov 2009 --=20 2.39.3