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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Mike Snitzer , Jens Axboe , Chuck Lever Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, Jeff Layton Subject: Re: [PATCH v7 3/3] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking In-Reply-To: <20260511-dontcache-v7-3-2848ddce8090@kernel.org> Date: Wed, 13 May 2026 08:31:23 +0530 Message-ID: References: <20260511-dontcache-v7-0-2848ddce8090@kernel.org> <20260511-dontcache-v7-3-2848ddce8090@kernel.org> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Jeff Layton writes: > dontcache-bench results (same host, T6F_SKL_1920GBF, 251 GiB RAM, > xfs on NVMe, fio io_uring): > > Buffered and direct I/O paths are unaffected by this patchset. All > improvements are confined to the dontcache path: > > Single-stream throughput (MB/s): > Before After Change > seq-write/dontcache 298 897 +201% > rand-write/dontcache 131 236 +80% > > Tail latency improvements (seq-write/dontcache): > p99: 135,266 us -> 23,986 us (-82%) > p99.9: 8,925,479 us -> 28,443 us (-99.7%) > > Multi-writer (4 jobs, sequential write): > Before After Change > dontcache aggregate (MB/s) 2,529 4,532 +79% > dontcache p99 (us) 8,553 1,002 -88% > dontcache p99.9 (us) 109,314 1,057 -99% > > Dontcache multi-writer throughput now matches buffered (4,532 vs > 4,616 MB/s). > > 32-file write (Axboe test): > Before After Change > dontcache aggregate (MB/s) 1,548 3,499 +126% > dontcache p99 (us) 10,170 602 -94% > Peak dirty pages (MB) 1,837 213 -88% > > Dontcache now reaches 81% of buffered throughput (was 35%). > > Competing writers (dontcache vs buffered, separate files): > Before After > buffered writer 868 433 MB/s > dontcache writer 415 433 MB/s > Aggregate 1,284 866 MB/s > > Previously the buffered writer starved the dontcache writer 2:1. > With per-bdi_writeback tracking, both writers now receive equal > bandwidth. The aggregate matches the buffered-vs-buffered baseline > (863 MB/s), indicating fair sharing regardless of I/O mode. > > The dontcache writer's p99.9 latency collapsed from 119 ms to > 33 ms (-73%), eliminating the severe periodic stalls seen in the > baseline. Both writers now share identical latency profiles, > matching the buffered-vs-buffered pattern. > > The per-bdi_writeback dirty tracking dramatically reduces peak dirty > pages in dontcache workloads, with the 32-file test dropping from > 1.8 GB to 213 MB. Dontcache sequential write throughput triples and > multi-writer throughput reaches parity with buffered I/O, with tail > latencies collapsing by 1-2 orders of magnitude. > Thanks for also considering multiple request for performance numbers into account, this indeed is a nice improvement overall to RWF_DONTCACHE. With Christian simplification - the patch looks good to me. So, please feel free to add: Reviewed-by: Ritesh Harjani (IBM)