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Thu, 29 May 2025 14:14:27 -0700 (PDT) Date: Thu, 29 May 2025 17:14:23 -0400 From: Johannes Weiner To: Matthew Wilcox Cc: Lorenzo Stoakes , Andrew Morton , Shakeel Butt , "Liam R . Howlett" , David Hildenbrand , Vlastimil Babka , Jann Horn , Arnd Bergmann , Christian Brauner , SeongJae Park , Usama Arif , Mike Rapoport , Barry Song <21cnbao@gmail.com>, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Pedro Falcato Subject: Re: [DISCUSSION] proposed mctl() API Message-ID: <20250529211423.GA1271329@cmpxchg.org> References: <85778a76-7dc8-4ea8-8827-acb45f74ee05@lucifer.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 1576980009 X-Stat-Signature: nttdu865y89djo71qstgrxa9eqidcqu6 X-Rspam-User: X-HE-Tag: 1748553268-332184 X-HE-Meta: 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 yuSGuYMz wXN40x9wD+mQjynqhp7AYC+Xc6BesdCKkUA+fKbcbr5pDMH9iRKFtl1XT8JWhgex6CY8lTyM49Hln1WukYFmjNR+tX8ZbFU/YZLsmfYJcWL1bH4VZbt8P+t4AaZlLH56Sn6pkZjcCkKqvOI9fH4WMG90JTKISS2GGJ+9rKl3b8BvSBc6VX9pBLg0r5Po0Gujzk0/7ygS1XD5MW4OtruTg2xeTfINn9wYTr5F2jAOTM5HnSdL8PZQ+hbYYDnB4RHSNGKBBr63Av2Xf9LY21j3apJcRDnfSJUGfNTRSVoc2dFpqQGhL/VHOL8WG+sXi/us2YMsQ1OkAWC1jPPHWNIpb4f4e/1NhmvQkks/sX/k6PWW9b+Bj+EHHXE8haT0AkIKPOM7vXX4RkFZzcKgV3LJ5i+agxiK2oVdjM14nbS5bZ53kptd8RWnrPbKiUqcSbY5WXbTz0W1iYOfSpqDA5NWv0djIyw== 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: List-Subscribe: List-Unsubscribe: On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote: > Barry's problem is that we're all nervous about possibly regressing > performance on some unknown workloads. Just try Barry's proposal, see > if anyone actually compains or if we're just afraid of our own shadows. I actually explained why I think this is a terrible idea. But okay, I tried the patch anyway. This is 'git log' on a hot kernel repo after a large IO stream: VANILLA BARRY Real time 49.93 ( +0.00%) 60.36 ( +20.48%) User time 32.10 ( +0.00%) 32.09 ( -0.04%) System time 14.41 ( +0.00%) 14.64 ( +1.50%) pgmajfault 9227.00 ( +0.00%) 18390.00 ( +99.30%) workingset_refault_file 184.00 ( +0.00%) 236899.00 (+127954.05%) Clearly we can't generally ignore page cache hits just because the mmaps() are intermittent. The whole point is to cache across processes and their various apertures into a common, long-lived filesystem space. Barry knows something about the relationship between certain processes and certain files that he could exploit with MADV_COLD-on-exit semantics. But that's not something the kernel can safely assume. Not without defeating the page cache for an entire class of file accesses.