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Fri, 10 Feb 2023 11:02:35 -0800 (PST) Date: Sat, 11 Feb 2023 03:02:17 +0800 From: Chih-En Lin To: Pasha Tatashin Cc: Andrew Morton , Qi Zheng , David Hildenbrand , "Matthew Wilcox (Oracle)" , Christophe Leroy , John Hubbard , Nadav Amit , Barry Song , Steven Rostedt , Masami Hiramatsu , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Yang Shi , Peter Xu , Vlastimil Babka , Zach O'Keefe , Yun Zhou , Hugh Dickins , Suren Baghdasaryan , Yu Zhao , Juergen Gross , Tong Tiangen , Liu Shixin , Anshuman Khandual , Li kunyu , Minchan Kim , Miaohe Lin , Gautam Menghani , Catalin Marinas , Mark Brown , Will Deacon , Vincenzo Frascino , Thomas Gleixner , "Eric W. Biederman" , Andy Lutomirski , Sebastian Andrzej Siewior , "Liam R. Howlett" , Fenghua Yu , Andrei Vagin , Barret Rhoden , Michal Hocko , "Jason A. Donenfeld" , Alexey Gladkov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Dinglan Peng , Pedro Fonseca , Jim Huang , Huichun Feng Subject: Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table Message-ID: References: <20230207035139.272707-1-shiyn.lin@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 9C5D9100032 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: x1qmn8nmpqbo5pzsags6noch8nznjchc X-HE-Tag: 1676055757-655454 X-HE-Meta: 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 DZKFdn8R 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 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: On Sat, Feb 11, 2023 at 01:20:10AM +0800, Chih-En Lin wrote: > On Fri, Feb 10, 2023 at 11:21:16AM -0500, Pasha Tatashin wrote: > > > > > Currently, copy-on-write is only used for the mapped memory; the child > > > > > process still needs to copy the entire page table from the parent > > > > > process during forking. The parent process might take a lot of time and > > > > > memory to copy the page table when the parent has a big page table > > > > > allocated. For example, the memory usage of a process after forking with > > > > > 1 GB mapped memory is as follows: > > > > > > > > For some reason, I was not able to reproduce performance improvements > > > > with a simple fork() performance measurement program. The results that > > > > I saw are the following: > > > > > > > > Base: > > > > Fork latency per gigabyte: 0.004416 seconds > > > > Fork latency per gigabyte: 0.004382 seconds > > > > Fork latency per gigabyte: 0.004442 seconds > > > > COW kernel: > > > > Fork latency per gigabyte: 0.004524 seconds > > > > Fork latency per gigabyte: 0.004764 seconds > > > > Fork latency per gigabyte: 0.004547 seconds > > > > > > > > AMD EPYC 7B12 64-Core Processor > > > > Base: > > > > Fork latency per gigabyte: 0.003923 seconds > > > > Fork latency per gigabyte: 0.003909 seconds > > > > Fork latency per gigabyte: 0.003955 seconds > > > > COW kernel: > > > > Fork latency per gigabyte: 0.004221 seconds > > > > Fork latency per gigabyte: 0.003882 seconds > > > > Fork latency per gigabyte: 0.003854 seconds > > > > > > > > Given, that page table for child is not copied, I was expecting the > > > > performance to be better with COW kernel, and also not to depend on > > > > the size of the parent. > > > > > > Yes, the child won't duplicate the page table, but fork will still > > > traverse all the page table entries to do the accounting. > > > And, since this patch expends the COW to the PTE table level, it's not > > > the mapped page (page table entry) grained anymore, so we have to > > > guarantee that all the mapped page is available to do COW mapping in > > > the such page table. > > > This kind of checking also costs some time. > > > As a result, since the accounting and the checking, the COW PTE fork > > > still depends on the size of the parent so the improvement might not > > > be significant. > > > > The current version of the series does not provide any performance > > improvements for fork(). I would recommend removing claims from the > > cover letter about better fork() performance, as this may be > > misleading for those looking for a way to speed up forking. In my > > From v3 to v4, I changed the implementation of the COW fork() part to do Sorry, it's "RFC v2 to v3". > the accounting and checking. At the time, I also removed most of the > descriptions about the better fork() performance. Maybe it's not enough > and still has some misleading. I will fix this in the next version. > Thanks. Thanks, Chih-En Lin