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by CH3PR17MB6241.namprd17.prod.outlook.com (2603:10b6:610:143::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7386.27; Tue, 19 Mar 2024 18:32:26 +0000 Received: from SJ0PR17MB5512.namprd17.prod.outlook.com ([fe80::6657:814f:5df0:bb5b]) by SJ0PR17MB5512.namprd17.prod.outlook.com ([fe80::6657:814f:5df0:bb5b%5]) with mapi id 15.20.7386.025; Tue, 19 Mar 2024 18:32:26 +0000 Date: Tue, 19 Mar 2024 14:32:17 -0400 From: Gregory Price To: Matthew Wilcox Cc: Gregory Price , linux-mm@kvack.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, ying.huang@intel.com, dan.j.williams@intel.com, honggyu.kim@sk.com, corbet@lwn.net, arnd@arndb.de, luto@kernel.org, akpm@linux-foundation.org, shuah@kernel.org Subject: Re: [RFC v3 3/3] ktest: sys_move_phys_pages ktest Message-ID: References: <20240319172609.332900-1-gregory.price@memverge.com> 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 OV6XCwi0 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information. > > > > > > What? LOL. No. > > > > > > > Certainly the test is stupid and requires admin, but I could not > > come up an easier test to demonstrate the concept - and the docs > > say to include a test with all syscall proposals. > > > > Am I missing something else important? > > (stupid question: of course I am, but alas I must ask it) > > It's not that the test is stupid. It's the concept that's stupid. Ok i'll bite. The 2 major ways page-hotness is detected right now is page-faults (induced or otherwise) and things like IBS/PEBS. page-faults cause overhead, and IBS/PEBS actually miss upwards of ~66% of all traffic (if you want the details i can dig up the presentation, but TL;DR: prefetcher traffic is missed entirely). so OCP folks have been proposing hotness-tracking offloaded to the memory devices themselves: https://www.opencompute.org/documents/ocp-cms-hotness-tracking-requirements-white-paper-pdf-1 (it's come along further than this white paper, but i need to dig up the new information). These devices are incapable of providing virtual addressing information, and doing reverse lookups of addresses is inordinately expensive from user space. This leaves: Do it all in a kernel task, or give user space an an interface to operate on data provided by the device. The syscall design is mostly being posted right now to collaborate via public channels, but if the idea is so fundamentally offensive then i'll drop it and relay the opinion accordingly. ~Gregory