Date: Wed, 01 Oct 2008 19:46:55 +0000 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Ed Schouten <ed@80386.nl> Cc: FreeBSD FS <freebsd-fs@freebsd.org>, Mark van Cuijk <mark@van-cuijk.nl>, Jille Timmermans <jille@quis.cx>, FreeBSD Arch <freebsd-arch@freebsd.org> Subject: Re: Expanding vops in vop_vectors during startup Message-ID: <69186.1222890415@critter.freebsd.dk> In-Reply-To: Your message of "Wed, 01 Oct 2008 21:07:28 %2B0200." <20081001190728.GL16837@hoeg.nl>
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In message <20081001190728.GL16837@hoeg.nl>, Ed Schouten writes: >The reason I'm sending this message, is because based on discussions I >had with several people on IRC we've basically got two different >opinions on this patch: > >- One group of people liked the idea of the patch. Some people even said > the patch looks good enough to be committed. > >- Another group of people also liked the idea, but thought it would make > no sense to commit it, because it's not like it's a bottleneck right > now. It should only be committed if an increase in performance is > notable. > >I did some tests with the patch set, by running tens of millions of >fstat(), fchown(), etc. calls to see how performance was affected. It >turns out on a kernel without any debugging options enabled, the >performance gain is only 1-2%, which sounds pretty valid to me. My resistance to this patch is not quite what you describe above: By factoring the vop vectors out, you remove the ability to let default vectors pick up new functionality later. I will admit that I have no knowledge of this level of generality, dating back from Heidemans Phd. dissertation, being used for anything sensible. Furthermore, if I am not mistaken, your patch increases the kernel size. Absent a plausible performance improvement, I don't see any point of your change. And that brings me to your "1-2%" measurement quoted in IRC and above. I have earlier ranted about the difference between benchmarking and random number generators, and you may have joined the project after the latest of these. Please search the mail-archives for that topic, and then use the handy ministat(1) program, to see if you have actually show any net speed benefit. Once and if you cross that threshold, I am going to raise my next objection: Benchmarking "tens of millions of fstat(), fchown(), etc. calls" and showing a 1-2% difference is patently bogus, and certainly no reason for the change you propose. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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