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Slow-tier Page Promotion discussion recap and open questions Cc: "Aneesh Kumar" , "David Hildenbrand" , "John Hubbard" , "Kirill Shutemov" , "Matthew Wilcox" , "Mel Gorman" , "Rao, Bharata Bhasker" , "Rik van Riel" , "RaghavendraKT" , "Wei Xu" , "Suyeon Lee" , "Lei Chen" , "Shukla, Santosh" , "Grimm, Jon" , , , "Liam Howlett" , "Gregory Price" , , "Kefeng Wang" To: "David Rientjes" , "Shivank Garg" From: "Zi Yan" X-Mailer: aerc 0.18.2 References: <6d582bb6-3ba5-1768-92f2-6025340a3cd4@google.com> <9093302B-95A9-4133-A0E0-75A47CE4336F@nvidia.com> <1c424899-d394-452f-9e13-d8cf77660c4a@amd.com> In-Reply-To: X-ClientProxiedBy: BN9PR03CA0655.namprd03.prod.outlook.com (2603:10b6:408:13b::30) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|PH0PR12MB5630:EE_ X-MS-Office365-Filtering-Correlation-Id: 6e3804af-8a83-468d-e9c6-08dd28f80695 X-MS-Exchange-SenderADCheck: 1 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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: List-Subscribe: List-Unsubscribe: On Mon Dec 30, 2024 at 12:30 AM EST, David Rientjes wrote: > On Thu, 19 Dec 2024, Shivank Garg wrote: > > > On 12/18/2024 8:20 PM, Zi Yan wrote: > > > On 17 Dec 2024, at 23:19, David Rientjes wrote: > > >=20 > > >> Hi everybody, > > >> > > >> We had a very interactive discussion last week led by RaghavendraKT = on > > >> slow-tier page promotion intended for memory tiering platforms, than= k > > >> you! Thanks as well to everybody who attended and provided great > > >> questions, suggestions, and feedback. > > >> > > >> The RFC patch series "mm: slowtier page promotion based on PTE A bit= "[1] > > >> is a proposal to allow for asynchronous page promotion based on memo= ry > > >> accesses as an alternative to NUMA Balancing based promotions. Ther= e was > > >> widespread interest in this topic and the discussion surfaced multip= le > > >> use cases and requirements, very focused on CXL use cases. > > >> > > > > > >> ----->o----- > > >> I asked about offloading the migration to a data mover, such as the = PSP > > >> for AMD, DMA engine, etc and whether that should be treated entirely > > >> separately as a topic. Bharata said there was a proof-of-concept > > >> available from AMD that does just that but the initial results were = not > > >> that encouraging. > > >> > > >> Zi asked if the DMA engine saturated the link between the slow and f= ast > > >> tiers. If we want to offload to a copy engine, we need to verify th= at > > >> the throughput is sufficient or we may be better off using idle cpus= to > > >> perform the migration for us. > > >=20 > > > > > >> > > >> - we likely want to reconsider the single threaded nature of the kt= hread > > >> even if only for NUMA purposes > > >> > > >=20 > > > Related to using DMA engine and/or multi threads for page migration, = I had > > > a patchset accelerating page migration[1] back in 2019. It showed goo= d > > > throughput speedup, ~4x using 16 threads to copy multiple 2MB THP. I = think > > > it is time to revisit the topic. > > >=20 > > >=20 > > > [1] https://lore.kernel.org/linux-mm/20190404020046.32741-1-zi.yan@se= nt.com/ > >=20 > > Hi All, > >=20 > > I wanted to provide some additional context regarding the AMD DMA offlo= ading > > POC mentioned by Bharata: > > https://lore.kernel.org/linux-mm/20240614221525.19170-1-shivankg@amd.co= m > >=20 > > While the initial results weren't as encouraging as hoped, I plan to im= prove this > > in next versions of the patchset. > >=20 > > The core idea in my RFC patchset is restructuring the folio move operat= ion > > to better leverage DMA hardware. Instead of the current folio-by-folio = approach: > >=20 > > for_each_folio() { > > copy metadata + content + update PTEs > > } > >=20 > > We batch the operations to minimize overhead: > >=20 > > for_each_folio() { > > copy metadata > > } > > DMA batch copy all content > > for_each_folio() { > > update PTEs > > } > >=20 > > My experiment showed that folio copy can consume up to 26.6% of total m= igration > > cost when moving data between NUMA nodes. This suggests significant roo= m for > > improvement through DMA offloading, particularly for the larger transfe= rs expected > > in CXL scenarios. > >=20 > > It would be interesting work on combining these approaches for optimize= d page > > promotion. > >=20 > > This is very exciting, thanks Shivank and Zi! The reason I brought this= =20 > topic up during the session on asynchronous page promotion for memory=20 > tiering was because page migration is likely going to become *much* more= =20 > popular and will be in the critical path under system-wide memory=20 > pressure. Hardware assist and any software optimizations that can go=20 > along with it would certainly be very interesting to discuss. > > Shivank, do you have an estimated timeline for when that patch series wil= l=20 > be refreshed? Any planned integration with TMPM? > > Zi, are you looking to refresh your series and continue discussing page=20 > migration offload? We could set up another Linux MM Alignment Session=20 > topic focused exactly on this and get representatives from the vendors=20 > involved. Sure. I am redoing the experiments with multithreads recently and see more throughput increase (up to 10x througput with 32 threads) on NVIDIA Grace CPUs. Shivank's approach, using MIGRATE_SYNC_NO_COPY, looks simpler than what I have done, splitting migrate_folio() into two parts[1]. I am planning to rebuild my multithreaded folio copy patches on top of Shivank's patches with some modifications. One thing to note is that MIGRATE_SYNC_NO_COPY is removed by Kefeng (cc'd) recently[2], so I will need to bring it back. [1] https://github.com/x-y-z/linux-dev/tree/batched_page_migration_copy-v6.= 12 [2] https://lore.kernel.org/all/20240524052843.182275-6-wangkefeng.wang@hua= wei.com/ --=20 Best Regards, Yan, Zi