Comment by scottlamb
2 days ago
> Disclaimer: An earlier version of this post claimed the structure is wait-free, this is incorrect. Being wait-free requires that failure or suspension of any thread can’t cause failure or suspension of another thread. This queue in fact does not fulfill that requirement. The main section which discusses the wait bounds of queue operations has been amended to reflect this, but other parts of this article have not been. As such there may parts of the text which refer to this as a wait-free queue, which it is not. I chose to keep those sections to avoid rewriting chunks of this post after it was already posted. Thanks for the correction Reddit user matthieum!
Classy disclaimer! matthieum's (long) reddit comment is also an informative read: https://www.reddit.com/r/rust/comments/1up0uhg/girls_just_wa...
Thanks. I jumped at the headline. I'd be happy with wait-free MPSC. I haven't checked in for a while. Have there been any breakthroughs in low-complexity wait-free queues in the past 10 years?
The closest thing I know of, is that there was a concurrent queue algo called LCRQ
It originally required double-width CAS, but IIRC in recent years someone figured out how to remove this to make it more portable
Best reference I could find from cursory google:
https://ppopp23.sigplan.org/details/PPoPP-2023-papers/2/The-...
https://nikitakoval.org/publications/ppopp23-lprq.pdf seems to be the paper in question.
I suspect the search space of low-complexity, or at least what I'd consider "low-complexity", wait-free queues is pretty much exhausted at this point.
The MPSC/MPMC structure in Aeron is wait-free with respect to producers - one producer cannot block another
There are simple node based algorithms that achieves a similar guarantee:
https://web.archive.org/web/20240928080729/https://www.1024c...
There is also a MPMC algorithm on this site very similar to the article
https://web.archive.org/web/20220524214823/https://www.1024c...
Nice to see Vyukov's MPMC queue mentioned. It's pretty neat. I have used a C implementation[1] of this in a small personal project.
[1]: https://github.com/dorjoy03/dsync/blob/master/src/mpmc_queue...
Both Vyukov queues are fast and useful in practice, but neither is even obstruction-free, let alone lock-free or wait-free.
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This paper [0] from 2022 is pretty good. "Low complexity" it is not, though.
[0] https://arxiv.org/pdf/2201.02179
Not a ton, the inexorable march to wider scalar dispatch, deeper pipelines, and ever less uniform geometries (I pine for the halcyon days of two NUMA modes with a QPI bar) has made asymmetrical fencing juicy enough to be be worth the squeeze at the margins, you weren't seeing a ton of `asm volatile ""` on one side and `membarrier()` on the other a decade ago and you'll see that now.
But I think Nathan Bronson's work out of IIRC Standford about 10 or 15 years ago is still more or less the canvas you paint on.