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Tue, 30 Aug 2022 07:40:09 -0700 (PDT) From: xu xin X-Google-Original-From: xu xin To: akpm@linux-foundation.org, adobriyan@gmail.com, willy@infradead.org Cc: bagasdotme@gmail.com, hughd@google.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, xu xin , Xiaokai Ran , Yang Yang Subject: [PATCH v5 2/2] ksm: add profit monitoring documentation Date: Tue, 30 Aug 2022 14:40:03 +0000 Message-Id: <20220830144003.299870-1-xu.xin16@zte.com.cn> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20220830143731.299702-1-xu.xin16@zte.com.cn> References: <20220830143731.299702-1-xu.xin16@zte.com.cn> MIME-Version: 1.0 ARC-Authentication-Results: i=1; imf10.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=YcB6giRB; spf=pass (imf10.hostedemail.com: domain of cgel.zte@gmail.com designates 209.85.210.170 as permitted sender) smtp.mailfrom=cgel.zte@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; 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dmarc=pass (policy=none) header.from=gmail.com X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1661870411-201110 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 Reviewed-by: Bagas Sanjaya --- 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..fb6ba2002a4b 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_pages * sizeof(page) - + ksm_rmap_items * sizeof(rmap_item). + + where ksm_merging_pages is shown under the directory ``/proc//``, + and ksm_rmap_items is shown in ``/proc//ksm_stat``. + +From the perspective of application, a high ratio of ``ksm_rmap_items`` to +``ksm_merging_pages`` 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_rmap_items/ksm_merging_pages`` 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 =====================