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[24.171.58.32]) by smtp.gmail.com with ESMTPSA id d25-20020a05683018f900b006e8aeb6706bsm340376otf.6.2024.04.05.09.49.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 05 Apr 2024 09:49:24 -0700 (PDT) From: Matthew Cassell To: corbet@lwn.net, akpm@linux-foundation.org, vbendel@redhat.com, rppt@kernel.org Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, mcassell411@gmail.com Subject: [PATCH] Documentation/admin-guide/sysctl/vm.rst adding the importance of NUMA-node count to documentation Date: Fri, 5 Apr 2024 16:49:20 +0000 Message-Id: <20240405164920.2844-1-mcassell411@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" If any bits are set in node_reclaim_mode (tunable via /proc/sys/vm/zone_reclaim_mode) within get_pages_from_freelist(), then page allocations start getting early access to reclaim via the node_reclaim() code path when memory pressure increases. This behavior provides the most optimization for multiple NUMA node machines. The above is mentioned in: Commit 9eeff2395e3cfd05c9b2e6 ("[PATCH] Zone reclaim: Reclaim logic") states "Zone reclaim is of particular importance for NUMA machines. It can be more beneficial to reclaim a page than taking the performance penalties that come with allocating a page on a REMOTE zone." While the pros/cons of staying on node versus allocating remotely are mentioned in commit histories and mailing lists. It isn't specifically mentioned in Documentation/ and isn't possible with a lone node. Imagine a situation where CONFIG_NUMA=3Dy (the default on most major distributions) and only a single NUMA node exists. The latter is an oxymoron (single-node =3D=3D uniform memory access). Informing the user via vm.rst t= hat the most bang for their buck is when multiple nodes exist seems helpful. Signed-off-by: Matthew Cassell --- Documentation/admin-guide/sysctl/vm.rst | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-= guide/sysctl/vm.rst index c59889de122b..10270548af2a 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -1031,7 +1031,8 @@ Consider enabling one or more zone_reclaim mode bits = if it's known that the workload is partitioned such that each partition fits within a NUMA node and that accessing remote memory would cause a measurable performance reduction. The page allocator will take additional actions before -allocating off node pages. +allocating off node pages. Keep in mind enabling bits in zone_reclaim_mode +makes the most sense for topologies consisting of multiple NUMA nodes. =20 Allowing zone reclaim to write out pages stops processes that are writing large amounts of data from dirtying pages on other nodes. Zone --=20 2.34.1