Girls just wanna have fast MPMC queues with bounded waiting

Girls just wanna have fast MPMC queues with bounded waiting(nahla.dev)

203 points by EvgeniyZh 89 days ago | 46 comments

scottlamb 86 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...

RossBencina 86 days ago | |

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?

gavinray 86 days ago | | |

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-...

duped 86 days ago | | |

This paper [0] from 2022 is pretty good. "Low complexity" it is not, though.

[0] https://arxiv.org/pdf/2201.02179

platinumrad 86 days ago | | |

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.

nly 86 days ago | | |

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...

reinitctxoffset 86 days ago | | |

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.

Human-Cabbage 86 days ago |

Surprised nobody here nor on the reddit thread caught this one: https://github.com/nahla-nee/wfqueue/blob/main/src/lib.rs#L3...

    unsafe impl<T, const N: usize> Sync for WFQueue<T, N> {}
    unsafe impl<T, const N: usize> Send for WFQueue<T, N> {}

These impls are unsound, because neither constrains `T` to be `Sync`/`Send`. As-written this would let you declare a `WFQueue<Rc<T>, N>` and pass non-atomic-refcount pointers between threads.

The fix is straightforward:

    unsafe impl<T: Sync, const N: usize> Sync for WFQueue<T, N> {}
    unsafe impl<T: Send, const N: usize> Send for WFQueue<T, N> {}

I.e., WFQueue is only Sync if T is Sync, and likewise for Send.

Actually, later on, the code makes a similar mistake, but only for one impl.

    unsafe impl<'a, T, const N: usize> Send for DrivableWFEnqueue<'a, T, N> where T: Send {}
    unsafe impl<'a, T, const N: usize> Send for DrivableWFDequeue<'a, T, N> {}

lpage 85 days ago | |

WFQueue isn't handing out &T, so Send is sufficient:

  unsafe impl<T: Send, const N: usize> Sync for WFQueue<T, N> {}
  unsafe impl<T: Send, const N: usize> Send for WFQueue<T, N> {}

rigtorp 86 days ago |

Here's my widely used implementation of this approach in C++: https://github.com/rigtorp/MPMCQueue

platinumrad 86 days ago | |

This was the very best bounded MPMC queue when I last looked into these things years ago, and as far as descendants of the Vyukov MPMC cycle queue go, I don't think it's possible to do much better.

I think your citation date is off, by the way. As far as I can tell, it was first published in January 2011.

duttish 86 days ago |

On the topic of lock free data structures I found this one on a SPSC very interesting too https://david.alvarezrosa.com/posts/optimizing-a-lock-free-r... taking it from 12M to 305M ops/s

rigtorp 86 days ago | |

That looks like a rewrite of my earlier work: https://rigtorp.se/ringbuffer/

andersced 85 days ago | |

https://github.com/andersc/fastqueue2/tree/main

RossBencina 86 days ago |

Perhaps I missed it but there didn't appear to be discussion of false sharing between the N individual data slots. It might be beneficial to pad each slot to a cache line width (or at least less slots per line), and/or using some kind of bijective hashing on the slot lookup so that sequential tickets don't access adjacent slots.

rigtorp 86 days ago | |

You would use one of those approaches:

If you align and pad each slot there won't be any false sharing and the stream prefetcher can kick in if there's only one producer or consumer.

If you use bijective hashing you reduce false sharing without aligning and padding. This can save memory at the expense of the stream prefetcher never kicking in.

ThePowerOfFuet 86 days ago |

>fast MPMC queues with bounded waiting

Sounds like fun.

xxs 86 days ago | |

It's very rare (if ever) I'd want M on both ends... and no work stealing. Usually it's one M and an S.