Re: git: 133935d26f20 - main - pf: atomically increment state ids

On 8 Oct 2022, at 23:26, Gleb Smirnoff 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 :(
> Previously two state creations could run in parallel without negatively
> affecting each other.
>
Slightly related: has anyone ever run a benchmark on state creation?
All of the benchmarking I’ve done so far focussed on traffic rates, so state lookups rather than state creation. (Which I think is the more important metric, but obviously we don’t want to overly pessimise state creation either.)

This change does buy us “we actually work on > 256 CPU machines”, and I strongly suspect this isn’t actually going to be the bottleneck on state creation either. Especially not once mjg improves unr64.

Best regards,
Kristof