Replace orx-concurrent-vec with append-only-vec

[MichaReiser]

An alternative to #554

append-only-vec performs even better than boxcar and also helps to reduce the overall dependencies (and runs on 32bit!).

[netlify]

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[codspeed-hq]

CodSpeed Performance Report

Merging #555 will improve performances by 12.16%

_{Comparing MichaReiser:append-only-vec (9fe3936) with master (d6df21f)}

Summary

⚡ 1 improvements

Benchmarks breakdown

	Benchmark	master	MichaReiser:append-only-vec	Change
⚡	many_tracked_structs	147 µs	131.1 µs	+12.16%

[nikomatsakis]

Seems like a winner!

[MichaReiser]

Thanks for fixing the version. I'll merge this tomorrow unless you wanted me to address something else.

[MichaReiser]

Okay I can't merge this while changes are requested. @nikomatsakis would you mind having another look.

[ibraheemdev]

FWIW append-only-vec isn't a great concurrent vector implementation as it relies heavily on spin locks. In particular, it holds a spin lock when allocating which is almost always a bad idea, but it also uses a spin lock to serialize insertion order. These not only hurt scalability but can also lead to unfortunate tail latency scenarios that are common with spin locks.

Of course, the upside is that array access is slightly cheaper (though looking closer this might not necessarily be the case because access always requires loading the contended length atomic) due to the extra ad-hoc synchronization, so if you expect writes to be relatively rare this may not be an issue.. but something to look out for.

[MichaReiser]

Thanks @ibraheemdev for the in-depth explanation.

I do think the spin-lock is fine the way we use it (at least for ingredients) because all write paths go through a lock anyway.

[ibraheemdev]

Interesting, a single-writer concurrent vector could be implemented more efficiently in general.. but probably not worth it for Salsa.