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 C16AC313E34; Sun, 26 Apr 2026 14:05:48 +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=1777212348; cv=none; b=MxfJoPqx4jO4sQJLLsNNoeJAoZ/SDIKpsixiRhrvWevXHd4moAG0qX5E7KIlDa85N9Qy9lf4BCxPLyNwS25a8vFmz06acwloJiT0BhOjRRDszGRQA5SbdQZ7OoLWnk1m6N98nGN8UTMHOjyNFEyH15aXrh6CjriXHSZXN115TQw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777212348; c=relaxed/simple; bh=T4tAq8f3GyMyfad25bLCniXA7N6Mz1oCdcVsZpFwgJ8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=FJ/XVSdDN20/qavuIu1o2wyhZP6Mx6OVzWWBCy4LtJHDNO9ERbViL7ZJrh+bW+5EgO3KkULWvtdvGfkdq8ZslqB21Tw55DhYjc0jR0V6SLQL8FpbNuMAZqMY/4vhx8EX89WlFjrZ9ce3WkYozqsRZzpEkAm0AFeAD+P5xXieVHw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GuZN3lNZ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GuZN3lNZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 79FB6C2BCAF; Sun, 26 Apr 2026 14:05:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777212348; bh=T4tAq8f3GyMyfad25bLCniXA7N6Mz1oCdcVsZpFwgJ8=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=GuZN3lNZME/yjE2qdULa20BWGzJC/R7ThVsMc3/fu4Nra77mXgLO8zF/akws1vuMR 5SahTXISJHdXrw34jbyROxbYk2kXjFD0DgG3REtg1X/z9rFdbvC2ZI9LyV3VC8DdTm rZNPiqoj/9gDCnQKCW4ACKQsWaMmF3iW/6COSdXeZclcYHfxE0haX/Y7F8RxjWgXby YHKOBnQGknEAmkSGPRHysFUiOmCEr7sC1C+rOOr7stT0u2bqpa7hbEFwYzfx1vL4p2 Wz42SCHeI5jshpWp6VH5zy8UKI2+Kh4XzYwcI5OYZsQJu6YQEOOBAOdvHWnnFFDHKZ iopiD24Wp+XyA== Message-ID: <2a2d77bb3caf096781b1a4a6000a1c1318d52840.camel@kernel.org> Subject: Re: [PATCH v3 2/4] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking From: Jeff Layton To: Andrew Morton 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 , Christoph Hellwig , Kairui Song , Qi Zheng , Shakeel Butt , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Chuck Lever , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Date: Sun, 26 Apr 2026 10:05:42 -0400 In-Reply-To: <20260426052854.8372fb9d4c616f16a8aa0a0f@linux-foundation.org> References: <20260426-dontcache-v3-0-79eb37da9547@kernel.org> <20260426-dontcache-v3-2-79eb37da9547@kernel.org> <20260426052854.8372fb9d4c616f16a8aa0a0f@linux-foundation.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sun, 2026-04-26 at 05:28 -0700, Andrew Morton wrote: > Naive questions... >=20 > On Sun, 26 Apr 2026 07:56:08 -0400 Jeff Layton wrote= : >=20 > > 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 =E2=80=94 walking the page tree and submitting I/O blocks t= he writer > > for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms > > (dontcache). >=20 > So in the current case, when generic_write_sync() returns, all that > memory is written back and clean&reclaimable (or freed?), yes? >=20 No. Before returning, it submits the I/Os for the portion that it wrote rather than leaving it to the flusher to take care of things, but it doesn't wait for the I/Os to complete. > > 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. >=20 > Whereas after this change, that pagecache is probably still dirty, > unreclaimable, waiting for the flusher to do its thing? >=20 Correct, but that's sort of the case today too since DONTCACHE I/Os don't wait for the completion. With this change we're just deferring the I/O submission to the flusher thread (which should hopefully soon wake and take care of business). If the flusher thread can't keep up, then eventually balance_dirty_pages() will kick in and start slowing things down. > So is there potential that the system will get all gummed up with > dirty, to-be-written-soon pagecache? Is there something which limits > this buildup? >=20 Today in this situation, the writers are limited by the backing device throughput. Once the I/O submission queues are full, then the DONTCACHE writers end up stacking up on those. With this change, the writers will be more limited by traditional VM limits in this situation.=20 In the test runs I did, the peak pagecache with DONTCACHE writes was higher than with the unpatched version but still considerably less than with normal buffered I/O. That's the cost of deferring the I/O submission to the flusher. One thing we could consider is going back to submitting the writes inline when the number of dirty pages is high. But, that could have a detrimental effect on performance too. > > ... > >=20 > > dontcache-bench results on dual-socket Xeon Gold 6138 (80 CPUs, 256 GB > > RAM, Samsung MZ1LB1T9HALS 1.7 TB NVMe, local XFS, io_uring, file size > > ~503 GB, compared to a v6.19-ish baseline): > >=20 > > Single-client sequential write (MB/s): > > baseline patched change > > buffered 1449.8 1440.1 -0.7% > > dontcache 1347.9 1461.5 +8.4% > > direct 1450.0 1440.1 -0.7% > >=20 > > Single-client sequential write latency (us): > > baseline patched change > > dontcache p50 3031.0 10551.3 +248.1% > > dontcache p99 74973.2 21626.9 -71.2% > > dontcache p99.9 85459.0 23199.7 -72.9% > >=20 > > Single-client random write (MB/s): > > baseline patched change > > dontcache 284.2 295.4 +3.9% > >=20 > > Single-client random write p99.9 latency (us): > > baseline patched change > > dontcache 2277.4 872.4 -61.7% > >=20 > > Multi-writer aggregate throughput (MB/s): > > baseline patched change > > buffered 1619.5 1611.2 -0.5% > > dontcache 1281.1 1629.4 +27.2% > > direct 1545.4 1609.4 +4.1% > >=20 > > Mixed-mode noisy neighbor (dontcache writer + buffered readers): > > baseline patched change > > writer (MB/s) 1297.6 1471.1 +13.4% > > readers avg (MB/s) 855.0 462.4 -45.9% >=20 > These results look ambiguous. Sometimes better, sometimes worse? >=20 > > nfsd-io-bench results on same hardware (XFS on NVMe, NFSv3 via fio > > NFS engine with libnfs, 1024 NFSD threads, pool_mode=3Dpernode, > > file size ~502 GB, compared to v6.19-ish baseline): > >=20 > > Single-client sequential write (MB/s): > > baseline patched change > > buffered 4844.2 4653.4 -3.9% > > dontcache 3028.3 3723.1 +22.9% > > direct 957.6 987.8 +3.2% > >=20 > > Single-client sequential write p99.9 latency (us): > > baseline patched change > > dontcache 759169.0 175112.2 -76.9% > >=20 > > Single-client random write (MB/s): > > baseline patched change > > dontcache 590.0 1561.0 +164.6% > >=20 > > Multi-writer aggregate throughput (MB/s): > > baseline patched change > > buffered 9636.3 9422.9 -2.2% > > dontcache 1894.9 9442.6 +398.3% > > direct 809.6 975.1 +20.4% > >=20 > > Noisy neighbor (dontcache writer + random readers): > > baseline patched change > > writer (MB/s) 1854.5 4063.6 +119.1% > > readers avg (MB/s) 131.2 101.6 -22.5% >=20 > Ditto but less so. >=20 > > The NFS results show even larger improvements than the local benchmarks= . > > Multi-writer dontcache throughput improves nearly 5x, matching buffered > > I/O. Dirty page footprint drops 85-95% in sequential workloads vs. > > buffered. >=20 > It sounds that you like the results, so OK ;) --=20 Jeff Layton