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 9576937C902 for ; Tue, 23 Jun 2026 13:33:29 +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=1782221610; cv=none; b=CFyEjW9J6jgswZYqJZM77lctHUjlO0x0pM/81y/hHJ9gXQfd8heoemW+3JBMtZo2GY+km6x38QwgZtcfirRLolz6qR3NJH20NeKc/b0/wWB78ufktZvqOzeCqUuZ0njely/1YYRcbQ1AzqtS9zs6hYK1AvJRt6ihXAC6VJH2d1E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782221610; c=relaxed/simple; bh=6iXyg16Q30bV0ih3FzslAqnB7a40qL70Rjwe+kppTZo=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ZUXCPm+wbbwZvRwsE7eqoXPXxp8o43Mp6qZJlfl4IKW7Kuy2Wy/ELwfvY93AqDsr8g8Ze6bSo4z/1TWQG2hLmHSRxqLj7UjA0Gx+K8tgZPHN4VE1W/CVRHdKJG8dens5mMS/qBYnXJg1C7TGXpv/M+Q27L58QakOSTV8ro9uOGw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eU/ShVbe; 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="eU/ShVbe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 770461F000E9; Tue, 23 Jun 2026 13:33:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782221609; bh=b66C/nu9vqMjImE8IOL4PUbeXsgTqJ9A5lqAx4/CVpg=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=eU/ShVbesfRvrAmoGITC79hVyHaLxvdC48/fzeTIiPiPAvXodihuv1KHv8FddhzdB S0QlYQOJ0BknEdnZoxmH36wmIiBscS5c0T4Mm4IiTtSNAnwQvumRysBmzdhlV0HZbF wUJeDFd6giqWL835Chgypgiyt3tQAVCInld/d+aB6XEvcS6S9V6iDu749M/1JnErS7 kgJmfK8uZWCstZCVxX4Sn01fNUqLTApcwDOaNakeg5TYeQZmAL8QRFAKgW0bheH6hi 74RZl9dLxGEUtupRZBvUVhexBcEUN1UIQJwdrQAICGp5HctxGch98QBiYqGr6dHl/J CFP3c2WGu1b5g== Message-ID: <6076ff478091a512354443e7764ca1b737a386aa.camel@kernel.org> Subject: Re: NFS delegations behavior analysis From: Jeff Layton To: Benjamin Coddington Cc: "Mkrtchyan, Tigran" , Piyush Sachdeva , linux-nfs , Chuck Lever , trondmy , sfrench@samba.org, sprasad@microsoft.com, vaibsharma@microsoft.com Date: Tue, 23 Jun 2026 09:33:26 -0400 In-Reply-To: <4323491E-64A0-4883-8343-5EB569A0D81A@hammerspace.com> References: <0b39c1e01a92f99fe456c76523ec7f3aa5dc1a81.camel@kernel.org> <455619640.1622514.1782212671358.JavaMail.zimbra@desy.de> <227140ce94ebe4186d02b081489a58f32b878ec4.camel@kernel.org> <4323491E-64A0-4883-8343-5EB569A0D81A@hammerspace.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw 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Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-06-23 at 09:11 -0400, Benjamin Coddington wrote: > On 23 Jun 2026, at 7:10, Jeff Layton wrote: >=20 > > 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 > I haven't (shame) thought about READDIR in the context of directory > delegations. >=20 > But during my time at Red Hat we worked hard to optimize some readdir > problems and I learned that almost any change we made ended up making > someone's workload regress. We also found that our performance benchmark= s > rarely matched the most common real-world workloads. We made the mistake= of > trying to improve the benchmark which resulted in performance regressions > for real-world users. >=20 > Jeff, you've already touched on the core issue regarding fixing this with > bulk GETATTR calls - the kernel doesn't know what syscall pattern the > userspace process is going to use next. The `ls -l` command and `find` a= nd > friends have complex history and branching logic, they do different looku= p > and getattr patterns based on their own goals, and NFS cannot optimize fo= r > any one case. >=20 > I think the last time we discussed additional improvements there were som= e > ideas about teaching the readdir code to respond to fadvise flags, but th= en > you'd also need to teach the utilities how to use them as well, and those > utilities try to be filesystem-agnostic. >=20 > Its a tough problem, and sometimes the simplest thing might be to just us= e > more directories on NFS. >=20 > **coffee!** >=20 > .... er - so with directory delegations, can we simply re-hydrate the den= try > cache from the directory page mappings if the delegation is still valid? > Does the directory delegation pin the mapping? Clearly I need to look at > the code.. >=20 I think the main problem is not the dcache, but the attributes on the inodes. The first pass uses a full-attrs READDIR (aka readdirplus) and everything goes reasonably quick. Then, on the second pass, the client skips checking directory attributes before trusting the dcache because of the delegation (which is good), but the inodes those dentries point to have an attrcache timeout, and we end up doing a GETATTR for each statx() call when a readdirplus would have been cheaper. I guess after you see a few statx() calls from the same pid on the same directory in a short timeframe you could switch to doing READDIR,=C2=A0but that sounds horrid to get right and keep working. --=20 Jeff Layton