Date: Sun, 9 Oct 2022 00:34:53 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Gleb Smirnoff <glebius@freebsd.org> Cc: Kristof Provost <kp@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 133935d26f20 - main - pf: atomically increment state ids Message-ID: <CAGudoHFoAOXNo5REmkAnhipTWZ3Uf2TofW4QTmOG%2BXgKDyA4tQ@mail.gmail.com> In-Reply-To: <Y0H4/IQ3STFvnm6m@FreeBSD.org> References: <202210081641.298GfT2F036984@gitrepo.freebsd.org> <Y0H4/IQ3STFvnm6m@FreeBSD.org>
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On 10/9/22, Gleb Smirnoff <glebius@freebsd.org> wrote: > On Sat, Oct 08, 2022 at 04:41:29PM +0000, Kristof Provost wrote: > K> commit 133935d26f20c0b9c433da9a2b32bcbe562bc2c5 > K> Author: Kristof Provost <kp@FreeBSD.org> > K> AuthorDate: 2022-10-07 17:17:06 +0000 > K> Commit: Kristof Provost <kp@FreeBSD.org> > K> CommitDate: 2022-10-08 16:27:29 +0000 > K> > K> pf: atomically increment state ids > K> > K> Rather than using a per-cpu state counter, and adding in the CPU id > we > K> can atomically increment the number. > K> This has the advantage of removing the assumption that the CPU ID > fits > K> in 8 bits. > K> > K> Event: Aberdeen Hackathon 2022 > K> Reviewed by: mjg > K> Differential Revision: https://reviews.freebsd.org/D36915 > > This adds an atomic operation on a single word on a state creation :( True, but patching unr64 to be fully scalable is not hard and I intend to do it later. The point from my end was to not add anything handrolled. > Previously two state creations could run in parallel without negatively > affecting each other. > That's only partially true. For one, the hash tables used (and associated locking) are not properly aligned to avoid false-sharing. But even if they were, you still have a lot of hashing conflicts and associated ping-pong from locking on lookup. And so on. I do have WIP work introduce lockless lookup here though, but there is technical debt to clean up. -- Mateusz Guzik <mjguzik gmail.com>
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