Put a ring on it: a lock-free MPMC ring buffer

by signa11 • 7 months ago

77 44 7 months ago

This isn't that new. (see: FASTER: A Concurrent Key-Value Store with In-Place Updates. 2018 ACM SIGMOD International Conference on Management of Data and related papers)

However, this is well written and very easy to read.

viega7• 7 months ago

Well, when I was doing the original work on it (about 5 years ago now), I spent a lot of time trying to find something else in the literature. I couldn't find anything that wasn't SPMC or MPSC, unless it had severe limitations, like not actually having a drop policy when full.

However, I definitely did not see the paper you've sited, but just spent a few minutes with the paper you cited. Section 5.2 seems to cover their version. It's far from clear what their algorithm is, but they are talking about page eviction; it doesn't seem like they're using even a single fixed array for the ring, but I'm not 100% sure because it's pretty light on any detail. a

binarycoffee• 7 months ago

I hope you won't mind my picking your brain: which MPSC ring buffer implementations did you find that does drop old items when full? I could only found implementations that are basically re-purposed MPMC, or that cannot deal with non-POD data (seqlock-based).

jcalvinowens• 7 months ago

This classic paper from 1996 describes a simple unbounded mpmc queue: https://www.cs.rochester.edu/~scott/papers/1996_PODC_queues....

auxym• 7 months ago

Also this Nim library: https://github.com/nim-works/loony

Which is based on: https://ieeexplore.ieee.org/document/9490347

viega7• 7 months ago

So that work came after mine, and seems to be a FIFO not a ring buffer. The library I built at the time also had FIFOs and LIFOs that were tweaks on previous algorithms, but nothing earth shaking. I'll check this one out when I can though.

JonChesterfield2• 7 months ago

The paper is annoyingly difficult to locate but the author's implementation is at https://github.com/oliver-giersch/looqueue-rs

WeaselNo7• 7 months ago

Strange to see a lock-free ring buffer without seeing mention of LMAX/Martin Thompson's Java Disruptor (https://github.com/LMAX-Exchange/disruptor)

viega7• 7 months ago

I don't think I even saw this until I published the article. I don't think it was academically published, or googled well back in 2000. Nor did it match my needs for a ring buffer at the time, which was to drop stale data (I think in that algorithm, write operations fail when the buffer is full), so if I did see it, I wouldn't have payed enough attention to notice if it even had users. It's good work for sure, but that's why it didn't get mentioned.

atq21192• 7 months ago

Eh, I get it. I also independently came up with these MPMC approaches before hearing about LMAX. They're not entirely trivial but they do follow fairly naturally from the problem space when you really think about it. It's a good piece of engineering, but the one thing that's really unique about LMAX is the amount of advertising it gets.

bob10292• 7 months ago

the one thing that's really unique about LMAX is the amount of advertising it gets.

Virtually zero? I have to go out of my way to remind HN it exists while everyone is 300+ comments deep into reinventing the wheel on a ~quarterly basis.

CyberDildonics• 7 months ago

There are a lot of algorithms that get passed around in java because they can deal only with pointers and pretend memory allocation and deallocation doesn't exist and doesn't lock. I'm not convinced it actually makes sense to use java as a reference language for lock free algorithms because it expose all the details.

qbane3• 7 months ago

I wrote my first SPSC circular buffer implementation in TS upon reading the previous post on futex [0] from the same author. It was more intricate than it had seemed, until I finally sat down and wrote my own code.

[0] https://h4x0r.org/futex/ discussion: https://news.ycombinator.com/item?id=44951563

xavierxwang3• 7 months ago

FYI: I have made a SPSC circular buffer for swap data in a pair of process: https://github.com/starwing/kaze-core

maybe that is what you want.

qbane3• 7 months ago

By a quick glance, yes, this is what I want: a channel to communicate between processes via a piece of shared memory, protected by a pair of futexes.

In JS ecosystem, buffers that allow data loss is more common (aka ring buffers), but ringbuf.js [1] is the only complete implementation to my knowledge. In my use case on I/O between WASM modules where data must be transferred as-is, the process must block on buffer overrun/underrun in a synchronous manner. Therefore a circular buffer is required. I could not find such a niche library written in JS, so I decided to bite the bullet and reinvent the wheel [2].

[1] https://github.com/padenot/ringbuf.js

[2] https://github.com/andy0130tw/spsc

bitexploder• 7 months ago

I recently started playing with these in game design as well to coordinate networking and game threads in a lock free manner. If it fits your use case it is truly a free lunch! They definitely have a lot of edge cases and are not easy to implement, but they aren't too crazy either.

atq21192• 7 months ago

Looks like a good write up, but I'd caution that some of the statements about memory models aren't completely accurate.

The terms relaxed, acquire, and release refer to how an atomic operation is ordered against other accesses to memory.

Counter to what the article states, a relaxed atomic is still atomic, meaning that it cannot tear and, for RMW atomic, no other access can go between the read and the write. But a relaxed atomic does not order other accesses, which can lead to unintuitive outcomes.

By contrast, once you've observed another thread's release store with an acquire load, you're guaranteed that your subsequent memory accesses "happen after" all of the other thread's accesses from before that release store -- which is what you'd intuitively expect, it's just that in modern systems (which are really highly distributed systems even on a single chip) there's a cost to establishing this kind of guarantee, which is why you can opt out of it with relaxed atomics if you know what you're doing.

viega7• 7 months ago

Yes, I meant to clarify the memory model discussion; I had tried to simplify and did a poor job; I got similar feedback after it was published, and never remembered to get to it. Will try to do it soon, though it's about the worst time for this to have hit, not sure when I'll be able to sit down for it, but will try to get it done in the next day. Hopefully it doesn't wait until next time it gets some views.

withinboredom2• 7 months ago

I also note that you don't mention the cache-line contention issue when accessing atomics in a multi-threaded context. That's a huge performance issue with lock-free constructs.

herodoturtle• 7 months ago

Co-founder of Crash Override.

Syzygies• 7 months ago

Yes. I was surprised there was no mention of "false sharing".

https://en.wikipedia.org/wiki/False_sharing

Rather than incrementing each counter by one, dither the counters to reduce cache conflicts? So what if the dequeue becomes a bit fuzzy. Make the queue a bit longer, so everyone survives at least as long as they would have survived before.

Or simply use a prime length queue, and change what +1 means, so one's stride is longer than the cache conflict concern. Any stride will generate the full cyclic group, for a prime.

gpderetta3• 7 months ago

My understanding is that there is no global ordering anyway: allocation of a node has a total order, but actually writing data to a node can be arbitrarily delayed. At this point might as well use separate queues for each writer.

edit: the author claims the algorithm is linearizable, so I'll keep reading the article.

edit2: Right, writer acquires ticket, when done attempts to set the done flag on node. Reader acquires ticket, attempts to consume the node. On failure the writer will retry. Even if you have one producer and one consumer, might not you, theoretically, end up in a scenario where the writer never make progress? I guess as technically no node was produced, this is still technically lock-free, but still... ... oh, I should have read it further, the single writer scenario (even in the dynamic case) is special cased! It will still produce the node.

Nice, but it seems exceedingly complex.

xavierxwang3• 7 months ago

Is there any chance to modify Vyukov's MPMC queue implement (https://www.1024cores.net/home/lock-free-algorithms/queues/b...) to support drop handler? That work doesn't need 128 bit CAS.

viega7• 7 months ago

You can use a 64-bit CAS if you want to use a 32-bit epoch and a pointer compression scheme of any kind, or just a 32-bit index into regions that are thread specific. I think I did the later when I did the original work, using the core primitive to build ring buffers that have arbitrary sized slots instead of 64-bit slots (which requires a bit of additional gymnastics, but the basic trick is to have the ring index into a bigger ring that you can FAA into, where the bigger ring has more slots by at least the max number of threads (I use this primitive heavily still for in-memory debug logging). Maybe at some point I'll do an article on that too.

viega7• 7 months ago

BTW, should be noted that the need to issue a cache line lock on x86 does seem to slow down 128-bit CAS quite a bit on x86-64 platforms. On arm64, there's no reason to skimp with a 64-bit CAS.

drob518• 7 months ago

Seems hung up on defining a ring buffer as something non-blocking (dropping). Having used ring buffers in both software and hardware systems for 40 years, we always called them ring buffers without this distinction. We would have called them all ring buffers. One nice thing about them is that it’s very easy to pass buffers back and forth between a single producer and single consumer without any locks or fancy memory atomics. All you need is a guarantee that memory writes occur in order. The AMD LANCE Ethernet controller (and many later derivatives) used this scheme to allow smooth overlapping of software frame processing with hardware transmission and reception way back in the 1980s.

loeg2• 7 months ago

Why would you want an MPMC queue primitive, instead of just using MPSC per consumer? I don't really see a discussion of the cache contention issues. (There is some mention of contention, but it seems the author is using the word in a different way.)

It looks like both enqueue and dequeue are RMWing the "epochs" variable? This can pretty easily lead to something like O(N^2) runtime if N processors are banging this cache line at the same time.

viega7• 7 months ago

For me, I’ve got use cases where it’s valuable to keep event data interleaved because it will also get used in flight. It works well enough I also use it for things where it’s not necessary like in memory debug rings (which requires a bit of additional work).

The epoch isn’t CAS’d; it is FAA. The epoch is then used to determine if there is contention due to the tail meeting the head, or due to a wrap-around due to slow writes.

There’s also a back-off scheme to ease contention for a full queue.

Though, I did originally have a variant that adds a fairly straightforward ‘help’ mechanism that makes the algorithm wait-free and reduces the computational complexity.

However, the extra overhead didn’t seem worth it, so I took it out pretty quickly. Iirc, the only place where the ring in the queue wouldn’t out-perform it are on tiny queues with a huge write imbalance.

If you go run the tests in the repo associated w the article, you probably will see that a ring with only 16 entries will tend to start being non-performant at about a 4:1 writer to reader ratio. But iirc that effect goes away before 128 slots in the ring.

There, the ring still fits in a page, and even with a big imbalance I can’t remember seeing less than 1m ops per second on my Mac laptop.

Real world observational data beats worst case analysis, and I’ve never seen an issue for scenarios I consider reasonable.

But, if my unrealistic is realistic to you, let me know and I can break out the wait free version for you.

loeg2• 7 months ago

The epoch isn’t CAS’d; it is FAA.

fetch_add requires taking a cache line exclusive. If producers and consumers are both doing this on the same cache line, it is very expensive and does not scale.

(withinboredom and gpderetta have raised the same or similar concerns.)

nly3• 7 months ago

A good write up, but a preallocated MPMC queue can be built without using CAS on every push and without fixed size slots and without multiple rings.

While even SPSC ring buffers are simple they are not particularly efficient in the case where the consumer is keeping up with the producer (the ideal case for a queue) due to all the cacheline ping pong. They are not the lowest latency solution

travisa• 7 months ago

For those of us not up to date with the state of the art, can you provide references? Very interested!

nly3• 7 months ago

Martin Thompson, of LMAX Disruptor fame, has I believe a novel solution in Aeron called the logbuffer, which fully formed 10+ years ago in that codebase.

gpderetta3• 7 months ago

[re weak vs strong cas] But, while that’s the guidance you’ll find all over the internet, I don’t actually know which CPUs this would affect. Maybe it’s old news, I dunno. But it does still seem to make a shade of difference in real-world tests

A CAS implemented with LL/SC (ARM, POWER) is weak as LL/SC an spuriously fail. So it always needs to be retried in a loop. Such a weak CAS might only be lock-free, not wait free as it might not provide global progress guarantees ; in practice some platforms give stronger progress guarantees as they might convert an LL/SC to a strong CAS via idiom recognition.

A strong CAS (x86, SPARC I thnk) is implemented directly in the architecture and it is typically strong. It also usually gives strong fairness guarantees.

If your algorithm needs to CAS in a loop might as well use a weak CAS to avoid a loop-of-loops. Otherwise a strong CAS might generate better code on some architectures.

32 bits is not enough space for the epoch if we are building something general-purpose.

Note that as long as your buffer can contain less than 31*2 items, 32 bits is usually enough (that's how TCP works for example) as even after overflow you can sequence before and after, unless you can have stale flight messages of more than one overflow ago.

However, the num_cells field and last_slot field are not tagged _Atomic. That’s because these should be set by one thread during initialization, and then never changed. As long as the memory has synced before other threads start to use these fields, we definitely don’t need them to be treated specially. Usually, if we do initialization in a proper function, the call boundary is going to be a memory barrier that makes sure they’re sync’d when other threads start getting a handle on our ring.

Your threading library likely guarantees that anything sequenced before the start of your thread happens-before the first instruction of the new thread is executed. So you do not need explicit memory barriers. In any case, a function call is at best a compiler barrier, not a full barrier as required on many architectures.

[sorry, I wasn't really going to do a review, these were my notes when reading the algo].

The algo is quite interesting, a lot of corner cases covered. The biggest issue is that the ticketing system is a single memory location where all producers and consumers attempt to write, so it is never going to scale.

If you really really need a lock-free MPMC queue that guarantees a total order, then it can be useful, but outside some hard-realtime scenarios, I can't really see the benefits. Wouldn't a collection of SPSC queues work for the logging scenario given in the introduction?

tialaramex• 7 months ago

From my reading of the article I think they understood why we'd want these two primitives for CAS, but they weren't clear (whereas your answer is better here) on whether that's a thing we care about today in 2025. ARM vs x86-64 matters for many people today whereas if we only wanted the other primitive for the M68k well, sorry Amiga fans but who cares.

Without immersion in the "Why" of each technological niche it can be hard to judge whether you're reading advice that really hasn't been relevant in decades ("The ASCII character set is not supported on all computers") or that's still important to your work today ("The file naming conventions may vary from one system to another")

gpderetta3• 7 months ago

whether that's a thing we care about today in 2025

Good question actually! ARM64 has a proper CAS it seems, but I think smaller ARMs still have only LL/SC and that's still relevant for embedded. I can't find a definitive answer for POWER, it is possible that is till only has LL/SC (I leave it to you whether POWER is still relevant, but certainly IBM cares that the standard support it, and it was still relevant during C++11 standardization). Can't find a definite answer for RISC-V: it seems that originally it only had LL/SC, but there are AMO extensions that add RISC-V.

I think most GPUs have native CAS instructions.

There are probably other embedded processors that are still relevant for C++, who knows what they support.

j_seigh• 7 months ago

I did a lock-free MPMC ring buffer with 1 128 bit CAS and 1 64 bit CAS for enqueue and 1 64 bit CAS for dequeue. The payload is an unrestricted uintptr_t (64 bit) value so no way to avoid the 128 bit CAS in the enqueue.

Put a ring on it: a lock-free MPMC ring buffer

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