From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 A235C3D0929 for ; Tue, 23 Jun 2026 10:50:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782211820; cv=none; b=UnveZ/+J/6CLVmi8nWx/vvG4qWVaZwGgji6RTAPIgfwMDNYOX5WU644vWEj2AdXEFR4XoIlqWhdSzL9OCQmOxoPF3Q9ww26jnhsvoL3nkXOoUnFqMtUvodxS4kwPjqyZHXaoXtbjM5w6SnaeJZHzqKFS8P64M9PiapidwHfoDLo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782211820; c=relaxed/simple; bh=6bArMXYcUZhAh4S7XlEzT8vS1ncbzin0YPfbxPfeODI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=bSgCXafvXFoAgs16R6Y7W9xwJgyKs7U/G6wEFXvcpfOLgrGzL/XFS+mxWojOSaGjo38Zqg3pYxWZFdu5S+5ULQ6wcqDHLEtWus2FgiahXbncdqgBpGdUgh2uPLy9IYofXw9Ew9hZRTnqWALmbdFA/19fh1awKdeIYqfhGLyJKLI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SFrpPXxx; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SFrpPXxx" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B0B801F00A3A; Tue, 23 Jun 2026 10:50:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782211819; bh=aSHiOnq2CdUo4BQyK7QmstkAG8aWffvDDm3hwxyGSV8=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=SFrpPXxxBLWTjUSgfBuGg8JR+VJoq5sTZNfOHSLBnaxMzSbYUXdt0zcMGueicqn8n e+pFuhk/zUC32BUzqrWhgdtISRn0R3PbPgNCtNcfaCzB4Q5UeuuCHa3dTaQ4ANTZdz B4YK0JF2UaTCunfCTXXbJivlUAjgVgqZNJVPUwiyRGGWqwQZLTSoaZWv8fBHa3g1yD Kz8anD9TxK32zVbDbxZ5TSPC/afMKAIuES+h50D2kkQNPyqLyqGQXtoCR+XwfL2LRC IMEwe4+tmuRXkX/qI9UVB0L7OCHv9v0d/aJvbaqIYErflXuypopZUB6/svyFxD7ZPy QgVHeABarR0nA== Message-ID: <0b39c1e01a92f99fe456c76523ec7f3aa5dc1a81.camel@kernel.org> Subject: Re: NFS delegations behavior analysis From: Jeff Layton To: Piyush Sachdeva , linux-nfs@vger.kernel.org, cel@kernel.org, trondmy@kernel.org, sfrench@samba.org, sprasad@microsoft.com Cc: vaibsharma@microsoft.com Date: Tue, 23 Jun 2026 06:50:16 -0400 In-Reply-To: References: Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV 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code documentation of the NFS client to better > understand the client side caching behavior with and without > delegations. >=20 > Understanding so far: > Delegations (both file and directory) are granted by the server to the > client, indefinitely (until revoked or under the watermark) to cache > attributes. The caching of data is a result of the attribute > cache. Hence forth, a directory delegation will cache the directory > attributes and the names of the files in the directory, and a file > delegation will cache the attributes of the file and the file data. >=20 > Workload run: > I focused on the 2 workloads below, doing 2 passes of a large flat > directory (with close to 100K files) -=20 > a cold pass, and warm pass using the cache from the cold pass: > - lslr - ls -lR on both runs > - lsmix - ls -R (cold) and then ls -lR (warm) >=20 > I also played with the rdirplus behavior using both the default > heuristic behavior and the `rdirplus=3Dforce` set at mount time. >=20 > Numbers: > actimeo=3D5s, rdirplus=3Dforce, ACLs off, flat_dir > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >=20 > | LSLR | LSMIX > | (ls -lR cold / warm) | (p1 ls -R / p2 ls -lR) > Operation | flat cold | flat warm | flat p1 | flat p2 > -----------------+-------------+-----------+-------------+--------- > READDIR calls | 27 | 0 | 27 | 0 > READDIR recv B | 23,603,024 | 0 | 23,603,024 | 0 > call type | readdirplus | -- | readdirplus | -- > LOOKUP | 1 | 0 | 1 | 0 > GETATTR | 3 | 100,000 | 2 | 100,001 > ACCESS | 2 | 0 | 2 | 0 > -----------------+-------------+-----------+-------------+--------- > Elapsed (age) | ~14 s | ~62 s | ~16 s | ~63 s >=20 >=20 > Observations: > When doing `ls` or `ls -l` on a directory, due to the open(2) on the > directory, the client gets a directory delegation - caching the > directory attributes and file names. However, as we don't have file > delegations due to no open(2) calls to any of the files. Henceforth, > the cache of file attributes is governed by `actimeo`. > Now here is the interesting bit, if the next `ls -l` is issued after > the `actimeo`, a massive GETATTR storm hits the server, doing stat() > calls for every file in the directory. As a result, the performance of > this warm `ls -l` run ends up being worse than the cold pass. I am > guessing this is most likely due to the compounded "rdirplus" being more > efficient than stat() calls. >=20 >=20 > Proposal: > For large directories, this ends up being a massive problem, taking 1-2 > minutes when enumerating a directory on the warm passes. > - An easier way to tackle this could be to do a rdirplus=3D[auto | forced= ] > instead of issuing the stat(2) storm to the server: When the client > notices that there are cache misses, which would be the case of file > attributes, instead of fetching file names from the directory-delegatio= n > cache and attributes from GETATTR, the client does a READDIRPLUS to > the server, nonetheless. > - A more tedious would be the to cache file attributes as well, as a part > of the directory delegation. This would end up requiring a change in th= e > NFS protocol spec though. > - Bulk GETATTR calls: I am uncertain of the feasibility of this, but > what if, the client could do 1 GETATTR call for getting attributes > for multiple files.=20 ls is such a hard workload to get right, because we don't really get an indication in the kernel of what userland's intentions are. It's basically a readdir() call followed by a bunch of stat()'s, but at the point where we're getting the readdir() call, we don't know if userland intends to stat() those files or not. We have to make a guess about that intention. In this case, it sounds like the directory cache was valid, so the client decided it didn't need to do a READDIR at all, but the individual files had caches that timed out. So imagine you're the kernel client and have been given that second readdir() call: Why should you decide to do a READDIRPLUS at that point instead of a regular READDIR? --=20 Jeff Layton