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 568B53D3008 for ; Tue, 23 Jun 2026 11:10:42 +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=1782213043; cv=none; b=t+RmGfR/pYyXdTjFe1N2DnLVjjobdctp0MXEyjnm4vvBbHPfHoBcUnZ8oWksvDo1IodHEVhVe4de/38RoKZ1uSMZ878FbGG7NOtQBTn0B5XponVjjhZzLGrZWYTafQ48R+I5AdQ94fgChnR//W9arj8fUwF/bFv4tFnTKEn5JEc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782213043; c=relaxed/simple; bh=NtcvFgG1VZiseA5clpiOuDwJWBYfQ+BqL9c/foYazeM=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=fsC3d6j6nIKhj+jFSfYln1Q16imEjomvNHL26z4wVv8aUET7KOjmE8StcjXEKCwrH3YdBFCWI+ZAtLWptg3BOxvPlRIUav3GzxYBL5r1vfOrGEG9KVkId05Nrp22si0sPv2GJ4oT9JjNaShPAtzfi+HGvFKcTPikfAE9L1oYUbA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mTzFhnBd; 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="mTzFhnBd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 105DF1F000E9; Tue, 23 Jun 2026 11:10:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782213041; bh=CwVKF+PFcvP+h9afhR16X+OCCJf/E4KGBejynDATNiQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=mTzFhnBd6jxX9b34c2rf4ImVrrMxmtVOmHuuSoQXOUU04zZUN1rFazzOSZ0Qf52uA TDd8CD4NzcEnhHJPI4PeUIf5PZu2naordRQLqCvFHAipmiqBHP7dLO8fUWnfv8toI4 Rvmxa6iCejmyAUeBZo4tcJW210ZyShuzB2ila6Guy9ct53pwc+iE8j1hxdwjPh6t9r 8iFJmLL06fbw0UVPlh2zAiF8wWqXF8+bKM9V6LRD2LUg2G27nX2tWBIrDKQe6CuyL0 RaGkoLlsU94/e3S/gJYZFnQ1zg2L93ED5Xd9BLR+qvtCw+7xjILd01cXOF0Olonkv1 GDHrBNh9EkruA== Message-ID: <227140ce94ebe4186d02b081489a58f32b878ec4.camel@kernel.org> Subject: Re: NFS delegations behavior analysis From: Jeff Layton To: "Mkrtchyan, Tigran" , Benjamin Coddington Cc: Piyush Sachdeva , linux-nfs , Chuck Lever , trondmy , sfrench@samba.org, sprasad@microsoft.com, vaibsharma@microsoft.com Date: Tue, 23 Jun 2026 07:10:39 -0400 In-Reply-To: <455619640.1622514.1782212671358.JavaMail.zimbra@desy.de> References: <0b39c1e01a92f99fe456c76523ec7f3aa5dc1a81.camel@kernel.org> <455619640.1622514.1782212671358.JavaMail.zimbra@desy.de> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Tue, 2026-06-23 at 13:04 +0200, Mkrtchyan, Tigran wrote: >=20 > ----- Original Message ----- > > From: "Jeff Layton" > > To: "Piyush Sachdeva" , "linux-nfs" , "Chuck Lever" , > > "trondmy" , sfrench@samba.org, sprasad@microsoft.co= m > > Cc: vaibsharma@microsoft.com > > Sent: Tuesday, 23 June, 2026 12:50:16 > > Subject: Re: NFS delegations behavior analysis >=20 > > On Tue, 2026-06-23 at 15:31 +0530, Piyush Sachdeva wrote: > > > Hi, > > > Lately I have been running micro benchmarks around the `ls` command a= nd > > > reading through the 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 th= e > > > 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) - > > > 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 o= f > > > 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 m= ore > > > 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 | fo= rced] > > > 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 fil= e > > > attributes, instead of fetching file names from the directory-deleg= ation > > > 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 i= n the > > > 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 > >=20 > > ls is such a hard workload to get right, because we don't really get an >=20 > 100% agree. And there were a couple of attempts to address this issue > (second ls that is slow). >=20 > > 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. > >=20 > > 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. > >=20 > > 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 > May we need some kind of client-side heuristics, like on the server side > for open-delegations, where after seeing some `stats` for files in the > In the same directory, the client will decide to switch to READDIR (v4) > to get all attributes in one go. >=20 Yeah, we definitely do. I'm just not sure what those heuristics look like. I think Ben did the latest pass of trying to tune the heuristics here. Any thoughts on how we could do this better (and whether there are particular ls-ish workloads that we don't want to regress)? --=20 Jeff Layton