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 A017F374E41; Fri, 27 Mar 2026 11:32:11 +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=1774611131; cv=none; b=DofKAmsfK2pYfIqvdMrhnq4ctcpg3f/BP6Q/6bxr2rVVefrYjqaEjA/lHHSZ/EyFAsnQGwrobNDb58UP+NHO5FQCbCRgl4PxYxvndeUahTYl8F0IGE96rMWnPRyG3XNrw0GDMFs9aM86+yYX0KzeulpkjtDXefgOxh0xx6K8fEE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774611131; c=relaxed/simple; bh=hl2sW+wBh2Iff5QX7aKEAM9AoJFEg75je/np73f8MCc=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Nuy7XjKWmixvCHs8lshXAMOTCC62b+X0uejyn9cxTsYO6p3wpJp2moO4e3vTKixjbE7i710oqVr0pgnEHOAMq7jBTMjsyGXlAnjx5UQ+styu7mfu18ETQus0DvbeEWxgG5XwbUgEnI61ilerCVQ54lsNTv/RzQdugAzt/YocQ3A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PiSYMBK0; 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="PiSYMBK0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 82422C19423; Fri, 27 Mar 2026 11:32:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774611131; bh=hl2sW+wBh2Iff5QX7aKEAM9AoJFEg75je/np73f8MCc=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=PiSYMBK0rcTRl3aWKPyGnp7uLdojNqQPXWatywdD88hHXz7NB1sTlLCbK9FzWUQgF oePXuEdZ8Ngz2eNDWYAh8cnevaH69zlnCjmzO/4cpLQCz6XkvAZw0xshZR0tKdSqWB 4Tpue6PvhQWrXH5N+kgOu4YsZaHAuzL4ZyN0FJjPGZwidYx4pGVkf0vzfXQKsc+5bx WrMMGjUjTSEnXwcox/SET7aGmdI49HNpW1iTcW260OZKja5+gMmYgFsUr3U5wEOGX8 TNCJwu04F9IrVgcMRt90y2FsKzwK/C1agBImvkPfhy49TEWhLiUWk3LzHm9ug0EvfI 7A8epZ0a+wwsg== Message-ID: <70e9c23a97d94a3dad5aa7f03f5a22c0950b00bf.camel@kernel.org> Subject: Re: A comparison of the new nfsd iomodes (and an experimental one) From: Jeff Layton To: Mike Snitzer Cc: Chuck Lever , linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, Jens Axboe Date: Fri, 27 Mar 2026 07:32:08 -0400 In-Reply-To: References: <4ebbd194ccfb3bcea6225d926b4c9f339e21c813.camel@kernel.org> <33582a86daf135336f6bc0d5260d8de0501abadd.camel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Thu, 2026-03-26 at 16:48 -0400, Mike Snitzer wrote: > On Thu, Mar 26, 2026 at 12:35:15PM -0400, Jeff Layton wrote: > > On Thu, 2026-03-26 at 11:30 -0400, Chuck Lever wrote: > > > On 3/26/26 11:23 AM, Jeff Layton wrote: > > > > I've been doing some benchmarking of the new nfsd iomodes, using > > > > different fio-based workloads. > > > >=20 > > > > The results have been interesting, but one thing that stands out is > > > > that RWF_DONTCACHE is absolutely terrible for streaming write > > > > workloads. That prompted me to experiment with a new iomode that ad= ded > > > > some optimizations (DONTCACHE_LAZY). > > > >=20 > > > > The results along with Claude's analysis are here: > > > >=20 > > > > https://markdownpastebin.com/?id=3D387375d00b5443b3a2e37d58a062= 331f > > > >=20 > > > > He gets a bit out over his skis on the upstream plan, but tl;dr is = that > > > > DONTCACHE_LAZY (which is DONTCACHE with some optimizations) outperf= orms > > > > the other write iomodes. > > >=20 > > > The analysis of the write modes seems plausible. I'm interested to he= ar > > > what Mike and Jens have to say about that. >=20 > Thanks for doing your testing and the summary, but I cannot help but > feel like your test isn't coming close to realizing the O_DIRECT > benefits over buffered that were covered in the past, e.g.: > https://www.youtube.com/watch?v=3DtpPFDu9Nuuw >=20 > Can Claude be made to watch a youtube video, summarize what it learned > and then adapt its test-plan accordingly? ;) >=20 I'm not sure it can. It's a good q though. I'll ask it! > Your bandwidth for 1MB sequential IO of 793 MB/s for O_DIRECT and > 4,952 MB/s for buffered and dontcache is considerably less than the 72 > GB/s offered in Jon's testbed. Your testing isn't exposing the > bottlenecks (contention) of the MM subsystem for buffered IO... not > yet put my finger on _why_ that is. That may very well be, but not everyone has a box as large as the one you and Jon were working with. > In Jon Flynn's testing he was using a working set of 312.5% of > available server memory, and the single client test system was using > fio with multiple threads and sync IO to write to 16 different mounts > (one per NVMe of the NFS server) with nconnect=3D16 and RDMA. >=20 This test was attempting to simulate a high nconnect count from multiple clients by using multiple fio processes. I also used the libnfs backend to fio so I wouldn't need to worry about tuning the kernel client. > Raw performance of a single NVMe in Jon's testbed was over 14 GB/s -- > he has the ability to drive 16 NVMe in his single NFS server. So an > order of magnitude more capable backend storage in Jon's NFS server. >=20 Very true. This box only had a single SSD. I can try to find something with more storage though for another test. > Big concern is your testing isn't exposing MM bottlenecks of buffered > IO... given that, its not really providing useful results to compare > with O_DIRECT. >=20 > Putting that aside, yes DONTCACHE as-is really isn't helpful.. your > lazy variant seems much more useful. >=20 Right. I think that's the big takeaway from this. Ignoring claude's rambling about changing default iomodes in the server, RWF_DONTCACHE just sucks for heavy writes. There is room for improvement there. The big question I have is whether fixing RWF_DONTCACHE's writeback behavior would give better results than what you were seeing with O_DIRECT. Do you guys still have access to that test rig? I can send you the patch if you want to test this and see how it does. > > > One thing I'd like to hear more about is why Claude felt that disabli= ng > > > splice read was beneficial. My own benchmarking in that area has show= n > > > that splice read is always a win over not using splice. > > >=20 > >=20 > > Good catch. That turns out to be a mistake in Claude's writeup. > >=20 > > The test scripts left splice reads enabled for buffered reads, and the > > results in the analysis reflect that. I (and it) have no idea why it > > would recommend disabling them, when the testing all left them enabled > > for buffered reads. >=20 > Claude had to have picked up on the mutual exclusion with splice_read > for both NFSD_IO_DONTCACHE and NFSD_IO_DIRECT io modes. So > splice_read is implicity disabled when testing NFSD_IO_DONTCACHE > (which is buffered IO). Yeah, Claude just got confused on the writeup. The benchmarking itself was sound, AFAICT. Buffered reads used splice and it was disabled for the other modes. --=20 Jeff Layton