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 115AB282F32; Sun, 26 Apr 2026 18:25:41 +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=1777227942; cv=none; b=EH44fiVRqtAa0MPvROrOnJQ79lrN3mbNFmJBoV/uSwj80gqXxqD6Q5kPI5+sKzARtnSLNHgEJNLrkLF1BhqZbyRW7i30jBoqkEF4x5QXPq6GLiUIFjRaEiMcBJsSOFfl+wzrswXAxqY7LQ8RqMqwf6xhh7lEqTp25C8NFRE72yg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777227942; c=relaxed/simple; bh=xAvxxc+GJErYibe3Dff2VWhiLf5NPpiX6PZCm4fg5XE=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=fDTZdgBuNlfN10y9Ks3Fx4j+/8btDHZEiaE7QPaGZeJED88QL5W/z07V+sAWrvmAJwLgcamzsliNNZ0akdBwtPXDWc/pKlCqqQBldZJBRE0J+JbcD93Uf/aCzeOaCIGlfuDvRWon1Kzanc2O6WUfWYJm+oN8VPufQCHxjcl8v5o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OlybZ2Iu; 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="OlybZ2Iu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A71ADC2BCAF; Sun, 26 Apr 2026 18:25:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777227941; bh=xAvxxc+GJErYibe3Dff2VWhiLf5NPpiX6PZCm4fg5XE=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=OlybZ2IujoiP0PzXbBns5uTh4PTxWaHeIyjJv3sUFL0Gs2b1QykWluRggP1n8afkT Tq64WvAky+PcSkCdPLQoi1XQKpzWaIE5S9Hq0bAgaNwjG/YAML+vcXZvB41M3Y+0MW 03N6Bxtmc7AttYs0TnoQMfCZVteNmubunXMc5EoEyLXxp/avrcMGqHQ7y2hgGVCEAm 65kY8DfTP/9i5W2XpsDILrABn5LvN6fDJFYgxnC+0gUS5d5t+wv8Ze9gSzyJfDgISS +HXIGCbeOz2z90jMgod1z2BzA7Gf4pyF9XvSEf92vW6XoiK1XI4yum3wwl6sFBzfxs AJsNfRtkS2ZZg== Message-ID: 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 14:25:36 -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 egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf CBYyUE8QJpvTZyfUHNYldXlrjO6n5MdOempLqWpfOmcGkwnyNRBR46g/jf8KnPRwXs509yAqDB6sE LZH+yWr9LQZEwARAQABtCVKZWZmIExheXRvbiA8amxheXRvbkBwb29jaGllcmVkcy5uZXQ+iQI7BB MBAgAlAhsDBgsJCAcDAgYVCAIJCgsEFgIDAQIeAQIXgAUCTpXWPAIZAQAKCRAADmhBGVaCFc65D/4 gBLNMHopQYgG/9RIM3kgFCCQV0pLv0hcg1cjr+bPI5f1PzJoOVi9s0wBDHwp8+vtHgYhM54yt43uI 7Htij0RHFL5eFqoVT4TSfAg2qlvNemJEOY0e4daljjmZM7UtmpGs9NN0r9r50W82eb5Kw5bc/r0km R/arUS2st+ecRsCnwAOj6HiURwIgfDMHGPtSkoPpu3DDp/cjcYUg3HaOJuTjtGHFH963B+f+hyQ2B rQZBBE76ErgTDJ2Db9Ey0kw7VEZ4I2nnVUY9B5dE2pJFVO5HJBMp30fUGKvwaKqYCU2iAKxdmJXRI ONb7dSde8LqZahuunPDMZyMA5+mkQl7kpIpR6kVDIiqmxzRuPeiMP7O2FCUlS2DnJnRVrHmCljLkZ Wf7ZUA22wJpepBligemtSRSbqCyZ3B48zJ8g5B8xLEntPo/NknSJaYRvfEQqGxgk5kkNWMIMDkfQO lDSXZvoxqU9wFH/9jTv1/6p8dHeGM0BsbBLMqQaqnWiVt5mG92E1zkOW69LnoozE6Le+12DsNW7Rj iR5K+27MObjXEYIW7FIvNN/TQ6U1EOsdxwB8o//Yfc3p2QqPr5uS93SDDan5ehH59BnHpguTc27Xi QQZ9EGiieCUx6Zh2ze3X2UW9YNzE15uKwkkuEIj60NvQRmEDfweYfOfPVOueC+iFifbQgSmVmZiBM YXl0b24gPGpsYXl0b25AcmVkaGF0LmNvbT6JAjgEEwECACIFAk6V0q0CGwMGCwkIBwMCBhUIAgkKC wQWAgMBAh4BAheAAAoJEAAOaEEZVoIViKUQALpvsacTMWWOd7SlPFzIYy2/fjvKlfB/Xs4YdNcf9q LqF+lk2RBUHdR/dGwZpvw/OLmnZ8TryDo2zXVJNWEEUFNc7wQpl3i78r6UU/GUY/RQmOgPhs3epQC 3PMJj4xFx+VuVcf/MXgDDdBUHaCTT793hyBeDbQuciARDJAW24Q1RCmjcwWIV/pgrlFa4lAXsmhoa c8UPc82Ijrs6ivlTweFf16VBc4nSLX5FB3ls7S5noRhm5/Zsd4PGPgIHgCZcPgkAnU1S/A/rSqf3F LpU+CbVBDvlVAnOq9gfNF+QiTlOHdZVIe4gEYAU3CUjbleywQqV02BKxPVM0C5/oVjMVx3bri75n1 TkBYGmqAXy9usCkHIsG5CBHmphv9MHmqMZQVsxvCzfnI5IO1+7MoloeeW/lxuyd0pU88dZsV/riHw 87i2GJUJtVlMl5IGBNFpqoNUoqmvRfEMeXhy/kUX4Xc03I1coZIgmwLmCSXwx9MaCPFzV/dOOrju2 xjO+2sYyB5BNtxRqUEyXglpujFZqJxxau7E0eXoYgoY9gtFGsspzFkVNntamVXEWVVgzJJr/EWW0y +jNd54MfPRqH+eCGuqlnNLktSAVz1MvVRY1dxUltSlDZT7P2bUoMorIPu8p7ZCg9dyX1+9T6Muc5d Hxf/BBP/ir+3e8JTFQBFOiLNdFtB9KZWZmIExheXRvbiA8amxheXRvbkBzYW1iYS5vcmc+iQI4BBM BAgAiBQJOldK9AhsDBgsJCAcDAgYVCAIJCgsEFgIDAQIeAQIXgAAKCRAADmhBGVaCFWgWD/0ZRi4h N9FK2BdQs9RwNnFZUr7JidAWfCrs37XrA/56olQl3ojn0fQtrP4DbTmCuh0SfMijB24psy1GnkPep naQ6VRf7Dxg/Y8muZELSOtsv2CKt3/02J1BBitrkkqmHyni5fLLYYg6fub0T/8Kwo1qGPdu1hx2BQ RERYtQ/S5d/T0cACdlzi6w8rs5f09hU9Tu4qV1JLKmBTgUWKN969HPRkxiojLQziHVyM/weR5Reu6 FZVNuVBGqBD+sfk/c98VJHjsQhYJijcsmgMb1NohAzwrBKcSGKOWJToGEO/1RkIN8tqGnYNp2G+aR 685D0chgTl1WzPRM6mFG1+n2b2RR95DxumKVpwBwdLPoCkI24JkeDJ7lXSe3uFWISstFGt0HL8Eew P8RuGC8s5h7Ct91HMNQTbjgA+Vi1foWUVXpEintAKgoywaIDlJfTZIl6Ew8ETN/7DLy8bXYgq0Xzh aKg3CnOUuGQV5/nl4OAX/3jocT5Cz/OtAiNYj5mLPeL5z2ZszjoCAH6caqsF2oLyAnLqRgDgR+wTQ T6gMhr2IRsl+cp8gPHBwQ4uZMb+X00c/Amm9VfviT+BI7B66cnC7Zv6Gvmtu2rEjWDGWPqUgccB7h dMKnKDthkA227/82tYoFiFMb/NwtgGrn5n2vwJyKN6SEoygGrNt0SI84y6hEVbQlSmVmZiBMYXl0b 24gPGpsYXl0b25AcHJpbWFyeWRhdGEuY29tPokCOQQTAQIAIwUCU4xmKQIbAwcLCQgHAwIBBhUIAg kKCwQWAgMBAh4BAheAAAoJEAAOaEEZVoIV1H0P/j4OUTwFd7BBbpoSp695qb6HqCzWMuExsp8nZjr uymMaeZbGr3OWMNEXRI1FWNHMtcMHWLP/RaDqCJil28proO+PQ/yPhsr2QqJcW4nr91tBrv/MqItu AXLYlsgXqp4BxLP67bzRJ1Bd2x0bWXurpEXY//VBOLnODqThGEcL7jouwjmnRh9FTKZfBDpFRaEfD FOXIfAkMKBa/c9TQwRpx2DPsl3eFWVCNuNGKeGsirLqCxUg5kWTxEorROppz9oU4HPicL6rRH22Ce 6nOAON2vHvhkUuO3GbffhrcsPD4DaYup4ic+DxWm+DaSSRJ+e1yJvwi6NmQ9P9UAuLG93S2MdNNbo sZ9P8k2mTOVKMc+GooI9Ve/vH8unwitwo7ORMVXhJeU6Q0X7zf3SjwDq2lBhn1DSuTsn2DbsNTiDv qrAaCvbsTsw+SZRwF85eG67eAwouYk+dnKmp1q57LDKMyzysij2oDKbcBlwB/TeX16p8+LxECv51a sjS9TInnipssssUDrHIvoTTXWcz7Y5wIngxDFwT8rPY3EggzLGfK5Zx2Q5S/N0FfmADmKknG/D8qG IcJE574D956tiUDKN4I+/g125ORR1v7bP+OIaayAvq17RP+qcAqkxc0x8iCYVCYDouDyNvWPGRhbL UO7mlBpjW9jK9e2fvZY9iw3QzIPGKtClKZWZmIExheXRvbiA8amVmZi5sYXl0b25AcHJpbWFyeWRh dGEuY29tPokCOQQTAQIAIwUCU4xmUAIbAwcLCQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJEAAOa EEZVoIVzJoQALFCS6n/FHQS+hIzHIb56JbokhK0AFqoLVzLKzrnaeXhE5isWcVg0eoV2oTScIwUSU apy94if69tnUo4Q7YNt8/6yFM6hwZAxFjOXR0ciGE3Q+Z1zi49Ox51yjGMQGxlakV9ep4sV/d5a50 M+LFTmYSAFp6HY23JN9PkjVJC4PUv5DYRbOZ6Y1+TfXKBAewMVqtwT1Y+LPlfmI8dbbbuUX/kKZ5d dhV2736fgyfpslvJKYl0YifUOVy4D1G/oSycyHkJG78OvX4JKcf2kKzVvg7/Rnv+AueCfFQ6nGwPn 0P91I7TEOC4XfZ6a1K3uTp4fPPs1Wn75X7K8lzJP/p8lme40uqwAyBjk+IA5VGd+CVRiyJTpGZwA0 jwSYLyXboX+Dqm9pSYzmC9+/AE7lIgpWj+3iNisp1SWtHc4pdtQ5EU2SEz8yKvDbD0lNDbv4ljI7e flPsvN6vOrxz24mCliEco5DwhpaaSnzWnbAPXhQDWb/lUgs/JNk8dtwmvWnqCwRqElMLVisAbJmC0 BhZ/Ab4sph3EaiZfdXKhiQqSGdK4La3OTJOJYZphPdGgnkvDV9Pl1QZ0ijXQrVIy3zd6VCNaKYq7B AKidn5g/2Q8oio9Tf4XfdZ9dtwcB+bwDJFgvvDYaZ5bI3ln4V3EyW5i2NfXazz/GA/I/ZtbsigCFc 8ftCBKZWZmIExheXRvbiA8amxheXRvbkBrZXJuZWwub3JnPokCOAQTAQIAIgUCWe8u6AIbAwYLCQg HAwIGFQgCCQoLBBYCAwECHgECF4AACgkQAA5oQRlWghUuCg/+Lb/xGxZD2Q1oJVAE37uW308UpVSD 2tAMJUvFTdDbfe3zKlPDTuVsyNsALBGclPLagJ5ZTP+Vp2irAN9uwBuacBOTtmOdz4ZN2tdvNgozz uxp4CHBDVzAslUi2idy+xpsp47DWPxYFIRP3M8QG/aNW052LaPc0cedYxp8+9eiVUNpxF4SiU4i9J DfX/sn9XcfoVZIxMpCRE750zvJvcCUz9HojsrMQ1NFc7MFT1z3MOW2/RlzPcog7xvR5ENPH19ojRD CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.3 (3.58.3-1.fc43) Precedence: bulk X-Mailing-List: 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 > > 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 > 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 > > ... > >=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 Forgot to comment on this part earlier... This is the "mixed-mode" (dontcache writes + buffered reads). I played with a bunch of different settings under nfsd, and those settings turned out to perform the best with this benchmark. I suspect what's happening is that the increase in write throughput from writing via the flusher thread is crowding out reads. So, read throughput suffers in this test from that. There are a number of ways we could probably make that more fair. > > 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 Same reason for the drop, I think. > > 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 ;) I think it's a win overall. As with anything writeback-related, it's a game of tradeoffs. The good news is that DONTCACHE is still fairly new and not many applications are using it yet, so the blast radius from any change here should be rather small. As a side note: I've long thought that we in general wait too long to kick off writeback with normal buffered I/O, particularly with modern memory sizes. DONTCACHE gives us a place to experiment with this scheme, but we may want to think about kicking off writeback earlier in the normal buffered case too. --=20 Jeff Layton