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?
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.
What's important is you know the trade-offs you are making.
You can't have a bounded queue that is always non-blocking because slow consumers can block producers.
You can't have a global FIFO order + multiple producers without slow producers blocking consumers.
You can't have a global FIFO order + have have non-atomic reserve and commit without a interrupted/de-scheduled producer thread being able to block the consumer
If you want atomic commit then you lose separate reserve which means either unbounded memory or atomic fixed-size data with sentinel values, ABA problems etc.
There are trade-offs everywhere, and it's best to pick the data structure that fits your needs just like any other problem.
> There are trade-offs everywhere, and it's best to pick the data structure that fits your needs just like any other problem.
That part I think is most crucial. Neither "Lock-free" nor "Wait-free" are vague terms for how awesome a thing is, they're specific properties which are expensive to provide, if you need such a property it was indispensable, if you don't need it then you can likely do better without it.
Exactly. I mentioned that those queues aren't formally obstruction-free because the context of the conversation was new developments in wait-free queues, even though I have only needed the guarantee once in my career and end up using descendants of the Vyukov MPMC cycle queue in practically all other cases because they are better on the metrics that count, like speed.
What was the one time when you need something wait free? I'm assuming interacting with hardware?
Hard real-time industrial automation. Worst job ever, by the way.
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.