From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id ECA2C12CD8B; Sat, 9 May 2026 00:20:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778286016; cv=none; b=WiTmwq4JHqGAqjfdfcGz/72cks7HaFCmW5aBfjiY2C/D0i6ea0n5zIXRv7HjWGxF7WteyJApB/w0H1Nbf1IMIM/ev0dJCGyTTpdUWzaR2A2RyRarAEeoaJ0F17rby8vyR4G/BtOy6YKXf/ZyS9tFUppAJOBs9AEeFwAjLTWJuKk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778286016; c=relaxed/simple; bh=5zM3V0nZSU9IeZvcOjJ3n2OX3NPDX1oav0rcmh7h2Xw=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=b2OIKFJQSuzjW+KVQifu5K6nR7y61h6TGXzKcwRwIH77QxMe4z6BysvqWx8NlXTdTpCNBispYjNuxyVO2g4a3ijy1LueTAj5AVlQGpNr4NsvxJhJjRNT1NOk0OKId3oohZKbGwjHFWp5ajeUcd1Xxto4hJlpzEDlKzxz2jyahNw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=t1dDhbvF; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="t1dDhbvF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DAA10C2BCB0; Sat, 9 May 2026 00:20:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1778286015; bh=5zM3V0nZSU9IeZvcOjJ3n2OX3NPDX1oav0rcmh7h2Xw=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=t1dDhbvFuFX23JsSoB5Lyv80NW8H46rIxg7hj0aDBu4xB9SceckWgig2M8wuG76G1 5eQFiMikbZQluIe36kjPj/7+1XLSgwrOxshHI8t1r4mkfSyNJtVksmt1bCysj8vEeR JbT7uThDJR1SwJJd4XLLeIWh2eBPfBHB5+TqZxPE= Date: Fri, 8 May 2026 17:20:14 -0700 From: Andrew Morton To: Jeff Layton Cc: Alexander Viro , Christian Brauner , Jan Kara , "Matthew Wilcox (Oracle)" , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Mike Snitzer , Jens Axboe , Ritesh Harjani , Chuck Lever , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v6 2/2] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking Message-Id: <20260508172014.8265ddc0220ff7e4d54674ff@linux-foundation.org> In-Reply-To: <20260505-dontcache-v6-2-66463805dd6a@kernel.org> References: <20260505-dontcache-v6-0-66463805dd6a@kernel.org> <20260505-dontcache-v6-2-66463805dd6a@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Tue, 05 May 2026 20:59:49 +0200 Jeff Layton wrote: > The IOCB_DONTCACHE writeback path in generic_write_sync() calls > filemap_flush_range() on every write, submitting writeback inline in > the writer's context. Perf lock contention profiling shows the > performance problem is not lock contention but the writeback submission > work itself — walking the page tree and submitting I/O blocks the writer > for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms > (dontcache). > > Replace the inline filemap_flush_range() call with a flusher kick that > drains dirty pages in the background. This moves writeback submission > completely off the writer's hot path. > > ... > > 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% > > 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 > > ... > > 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. Geeze, is that the best you can do ;) Sashiko seems to have found more stuff: https://sashiko.dev/#/patchset/20260505-dontcache-v6-0-66463805dd6a@kernel.org