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 1B416BE5E; Sat, 5 Jul 2025 11:33:00 +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=1751715181; cv=none; b=r6M1QfWxSAJCOwjwllBSB2Kv86bixTSeREfeW2Gq9Px7dOGCxvMLZPjRgRa+04aoXs1QCodTUJkfq/YTI/uafQ3Q7dA7xld7i+bUYVGzbKRuRwQte6TseTGOtEqHcx283+bb3D2/s38Xybi2vjkODVnelLt0n5HXWjhScfpR2wA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751715181; c=relaxed/simple; bh=i9Wq09osK8xb4YnzqvQDfeGZEgMDZE720RClxr++qOg=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=npLz+O09FEuYctnFmeADZwJlwPOrPlItGqpud9JvaECij5XiOBBFDbW59XW2hIdpL4q8NnF4jbfn0J5buSmdYj6z1YkPUl8PenY+Pxdnqsn3Acno1kKp1VL8aWAOtSN+axHgthwtlbW/ALaaCNUW+JVSrtCrPAIrg39t6eLFxgc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Sq2XZydh; 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="Sq2XZydh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BBBF3C4CEE7; Sat, 5 Jul 2025 11:32:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1751715180; bh=i9Wq09osK8xb4YnzqvQDfeGZEgMDZE720RClxr++qOg=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Sq2XZydh3dBN3IxLAMxmAjQmovFZJl6+TNVb02yAOFEK00PWk5TnDxV0m+2qfVtQb QmDG1jdPjROyIps8hQHKnNqTqfOGtlU/ocB2jfM4jHiADnjBZGOFbsY3f//FJJCMfj NtK/nHxQ1PMCf0j082e3+lxnqJJ8ChwpBiS5M4fmVVcr0PEHTSbXCYsXVZip8ChPy8 yjodNm+AR1tg+61d3rjj/Mjqk5Ch42hs0PBj1MTTVDhWOb/Vwl88JMiC+LS9RY4vMd bVUKgHGDVnFWzg9JKKrWXoLJrvMGZ35FLtgMC1wqehWUs8J83bgxfKDZYgD+xuCtTu 6tT9kZYYKpAug== Message-ID: Subject: Re: [PATCH RFC 0/2] nfsd: issue POSIX_FADV_DONTNEED after READ/WRITE/COMMIT From: Jeff Layton To: NeilBrown Cc: Trond Myklebust , Anna Schumaker , Chuck Lever , Olga Kornievskaia , Dai Ngo , Tom Talpey , Mike Snitzer , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Sat, 05 Jul 2025 07:32:58 -0400 In-Reply-To: <175158460396.565058.1455251307012063937@noble.neil.brown.name> References: <20250703-nfsd-testing-v1-0-cece54f36556@kernel.org> <175158460396.565058.1455251307012063937@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Fri, 2025-07-04 at 09:16 +1000, NeilBrown wrote: > On Fri, 04 Jul 2025, Jeff Layton wrote: > > Chuck and I were discussing RWF_DONTCACHE and he suggested that this > > might be an alternate approach. My main gripe with DONTCACHE was that i= t > > kicks off writeback after every WRITE operation. With NFS, we generally > > get a COMMIT operation at some point. Allowing us to batch up writes > > until that point has traditionally been considered better for > > performance. >=20 > I wonder if that traditional consideration is justified, give your > subsequent results. The addition of COMMIT in v3 allowed us to both: > - delay kicking off writes > - not wait for writes to complete >=20 > I think the second was always primary. Maybe we didn't consider the > value of the first enough. > Obviously the client caches writes and delays the start of writeback. > Adding another delay on the serve side does not seem to have a clear > justification. Maybe we *should* kick-off writeback immediately. There > would still be opportunity for subsequent WRITE requests to be merged > into the writeback queue. >=20 That is the fundamental question: should we delay writeback or not? It seems like delaying it is probably best, even in the modern era with SSDs, but we do need more numbers here (ideally across a range of workloads). > Ideally DONTCACHE should only affect cache usage and the latency of > subsequence READs. It shouldn't affect WRITE behaviour. >=20 It definitely does affect it today. The ideal thing IMO would be to just add the dropbehind flag to the folios on writes but not call filemap_fdatawrite_range_kick() on every write operation. After a COMMIT the pages should be clean and the vfs_fadvise call should just drop them from the cache, so this approach shouldn't materially change how writeback behaves. --=20 Jeff Layton