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Thu, 02 Apr 2026 03:20:00 -0700 (PDT) Received: from redhat.com ([209.132.188.88]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b2749cada2sm22687075ad.71.2026.04.02.03.19.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Apr 2026 03:19:59 -0700 (PDT) Date: Thu, 2 Apr 2026 18:19:56 +0800 From: Li Wang To: Michal =?utf-8?Q?Koutn=C3=BD?= Cc: Waiman Long , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Tejun Heo , Shuah Khan , Mike Rapoport , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Sean Christopherson , James Houghton , Sebastian Chlad , Guopeng Zhang , Li Wang Subject: Re: [PATCH v2 1/7] memcg: Scale up vmstats flush threshold with int_sqrt(nr_cpus+2) Message-ID: References: <20260320204241.1613861-1-longman@redhat.com> <20260320204241.1613861-2-longman@redhat.com> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: > > with kernel functions: > > var1 = 65536*nr_cpus / (45426 * ilog2(nr_cpus) + 65536) > > var2 = DIV_ROUND_UP(65536*nr_cpus, 45426 * ilog2(nr_cpus) + 65536) > > var3 = roundup_pow_of_two(var2) Consider a 1024-CPU machine with a cpuset-constrained cgroup using only 2 CPUs. Its unavoidable batching error is just 2MB, yet the global threshold imposes 256MB (harmonic-mean) or 32MB (sqrt) of additional error — 128x and 16x overprovisioning respectively. Both overshoot, but sqrt stays bit closer to the ideal. -- Regards, Li Wang