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Wed, 24 Aug 2022 00:08:26 -0700 (PDT) From: xu xin X-Google-Original-From: xu xin To: akpm@linux-foundation.org, corbet@lwn.net Cc: bagasdotme@gmail.com, adobriyan@gmail.com, willy@infradead.org, hughd@google.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, xu xin , Xiaokai Ran , Yang Yang Subject: [PATCH v3 2/2] ksm: add profit monitoring documentation Date: Wed, 24 Aug 2022 07:08:21 +0000 Message-Id: <20220824070821.220092-1-xu.xin16@zte.com.cn> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20220824070559.219977-1-xu.xin16@zte.com.cn> References: <20220824070559.219977-1-xu.xin16@zte.com.cn> MIME-Version: 1.0 ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=LEEMGQDs; spf=pass (imf04.hostedemail.com: domain of cgel.zte@gmail.com designates 209.85.210.174 as permitted sender) smtp.mailfrom=cgel.zte@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; 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spf=pass (imf04.hostedemail.com: domain of cgel.zte@gmail.com designates 209.85.210.174 as permitted sender) smtp.mailfrom=cgel.zte@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-HE-Tag: 1661324907-706931 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: Add the description of KSM profit and how to determine it separately in system-wide range and inner a single process. Signed-off-by: xu xin Reviewed-by: Xiaokai Ran Reviewed-by: Yang Yang --- Documentation/admin-guide/mm/ksm.rst | 36 ++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst index b244f0202a03..40bc11f6fa15 100644 --- a/Documentation/admin-guide/mm/ksm.rst +++ b/Documentation/admin-guide/mm/ksm.rst @@ -184,6 +184,42 @@ The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the ``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must be increased accordingly. +Monitoring KSM profit +===================== + +KSM can save memory by merging identical pages, but also can consume +additional memory, because it needs to generate a number of rmap_items to +save each scanned page's brief rmap information. Some of these pages may +be merged, but some may not be abled to be merged after being checked +several times, which are unprofitable memory consumed. + +1) How to determine whether KSM save memory or consume memory in system-wide +range? Here is a simple approximate calculation for reference: + + general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) * + sizeof(rmap_item); + +where all_rmap_items can be easily obtained by summing ``pages_sharing``, +``pages_shared``, ``pages_unshared`` and ``pages_volatile``. + +2) The KSM profit inner a single process can be similarly obtained by the +following approximate calculation: + + process_profit =~ ksm_merging_sharing * sizeof(page) - + ksm_rmp_items * sizeof(rmap_item). + +where both ksm_merging_sharing and ksm_rmp_items are shown under the directory +``/proc//``. + +From the perspective of application, a high ratio of ``ksm_rmp_items`` to +``ksm_merging_sharing`` means a bad madvise-applied policy, so developers or +administrators have to rethink how to change madvise policy. Giving an example +for reference, a page's size is usually 4K, and the rmap_item's size is +separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture. +so if the ``ksm_rmp_items/ksm_merging_sharing`` ratio exceeds 64 on 64-bit CPU +or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped, +because the ksm profit is approximately zero or negative. + Monitoring KSM events =====================