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Howlett" To: Matthew Wilcox Cc: "Christoph Lameter (Ampere)" , Mathieu Desnoyers , Sweet Tea Dorminy , Mateusz Guzik , linux-kernel@vger.kernel.org, Andrew Morton , "Paul E. McKenney" , Steven Rostedt , Masami Hiramatsu , Dennis Zhou , Tejun Heo , Martin Liu , David Rientjes , christian.koenig@amd.com, Shakeel Butt , Johannes Weiner , Lorenzo Stoakes , Suren Baghdasaryan , Vlastimil Babka , Christian Brauner , Wei Yang , David Hildenbrand , Miaohe Lin , Al Viro , linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, Yu Zhao , Roman Gushchin Subject: Re: [RFC PATCH v2] Introduce Hierarchical Per-CPU Counters Message-ID: Mail-Followup-To: "Liam R. Howlett" , Matthew Wilcox , "Christoph Lameter (Ampere)" , Mathieu Desnoyers , Sweet Tea Dorminy , Mateusz Guzik , linux-kernel@vger.kernel.org, Andrew Morton , "Paul E. 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 QX15eGGz 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000930, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: * Matthew Wilcox [250408 13:03]: > On Tue, Apr 08, 2025 at 09:37:18AM -0700, Christoph Lameter (Ampere) wrote: > > > The hierarchical per-CPU counters propagate a sum approximation through > > > a binary tree. When reaching the batch size, the carry is propagated > > > through a binary tree which consists of log2(nr_cpu_ids) levels. The > > > batch size for each level is twice the batch size of the prior level. > > > > A binary tree? Could we do this N-way? Otherwise the tree will be 8 levels > > on a 512 cpu machine. Given the inflation of the number of cpus this > > scheme better work up to 8K cpus. > > I find that a fan-out somewhere between 8 and 16 works well in practice. > log16(512) gives a 3 level tree as does a log8 tree. log16(8192) is a 4 > level tree whereas log8(8192) is a 5 level tree. Not a big difference > either way. > > Somebody was trying to persuade me that a new tree type that maintained > additional information at each level of the tree to make some operations > log(log(N)) would be a better idea than a B-tree that is log(N). I > countered that a wider tree made the argument unsound at any size tree > up to 100k. And we don't tend to have _that_ many objects in a > data structure inside the kernel. I still maintain vEB trees are super cool, but I am glad we didn't try to implement an RCU safe version. > > ceil(log14(100,000)) = 5 > ceil(log2(log2(100,000))) = 5 > > at a million, there's actually a gap, 6 vs 5. But constant factors > become a much larger factor than scalability arguments at that point. In retrospect, it seems more of a math win than a practical win - and only really the O(n) bounds. Beyond what willy points out, writes rippling up the tree should be a concern for most users since it will impact the restart of readers and negatively affect the writer speed - but probably not here (hot plug?). Working in (multiples of) cacheline sized b-tree nodes makes the most sense, in my experience. Thanks, Liam