In this post, I walk you step by step through implementing a single-producer single-consumer queue from scratch.
This pattern is widely used to share data between threads in the lowest-latency environments.
I guess the code samples inside post are under https://david.alvarezrosa.com/LICENSE
But feel free to ping me if you need different license, quite open about it
A lot of people focus on the code and then assume the device in question is only there to run it. There's so much you can tweak. I don't always measure it, but last time I saw at least a 20% improvement in Network throughput just by tweaking a few things on the machine.
What else could be improved? Would like to learn :)
Maybe using huge pages?
Disabling c-states, pinning network interfaces to dedicated cores (and isolating your application from those cores) and `SCHED_FIFO` (chrt -f 99 <prog>) helps a lot.
Transparent hugepages increase latency without you being aware of when it happens, I usually disable that.
Idk, there's a bunch but they all depend on your use-case. For example I always disable hyperthreading because I care more about latency than processing power- and I don't want to steal cache from my workload randomly.. but some people have more I/O bound workloads and hyperthreading is just and strict improvement in those situations.
In prod most trading companies do disable it, not sure about generic benchmarks best practices
It seems that in this case as you get contention the faster end will slow down (as it is consuming what the other end just read) and this will naturally create a small buffer and run at good speeds.
The hard part is probably that sentinel and ensuring that it can be set/cleared atomically. On Rust you can do `Option<T>` to get a sentinel for any type (and it very often doesn't take any space) but I don't think there is an API to atomically set/clear that flag. (Technically I think this is always possible because the sentinel that Option picks will always be small even if the T is very large, but I don't think there is an API for this.)
This was already the case with the cached index design at the end of the article, though. (Which doesn't require extra space or extra atomic stores.)
Would you mind expanding on the correctness guarantees enforced by the atomic semantics used? Are they ensuring two threads can't push to the same slot nor pop the same value from the ring? These type of atomic coordination usually comes from CAS or atomic increment calls, which I'm not seeing, thus I'm interested in hearing your take on it.
> note that there are only one consumer and one producer
That clarify things as you don't need multi-thread coordination on reads or writes if assuming single producer and single consumer.
- One single producer thread
- One single consumer thread
- Fixed buffer capacity
So to answer
> Are they ensuring two threads can't push to the same slot nor pop the same value from the ring?
No need for this usecase :)
Which is a slight shame since Load-Linked/Store-Conditional is pretty cool, but I guess that's limited to ARM anyways, and now they've added extensions for CAS due to speed.
Things get interesting when you're working with a cpu that lacks the ldrex/strem assembly instructions that makes this all work. I think youre only options at that point are disable/enable interrupts. IF anyone has any insights into this constraint I'd love to hear it.
In reality multiple threads for single core doesn't make much sense right?
https://en.cppreference.com/w/cpp/atomic/atomic/compare_exch...
The exact syntax and naming will of course differ, but any language that exposes low-level atomics at all is going to provide a pretty similar set of operations.
Sure, C++ has a particular way of describing atomics in a cross-platform way, but the actual hardware operations are not specific to the language.
But at the hardware level all are kindof the same
Also doesn't have fences on the store, has extra branches that shouldn't be there, and is written in really stylistically weird c++.
Maybe an llm that likes a different language more, copying a broken implementation off github? Mostly commenting because the initial replies are "best" and "lol", though I sympathise with one of those.
Are we reading the same code? The stores are clearly after value accesses.
> Also doesn't have fences on the store
?? It uses acquire/release semantics seemingly correctly. Explicit fences are not required.
buffer_[head] = value;
head_.store(next_head, std::memory_order_release);
return true;
There's no relationship between the two written variables. Stores to the two are independent and can be reordered. The aq/rel applies to the index, not to the unrelated non-atomic buffer located near the index.
No, this is incorrect. If you think there's no relationship, you don't understand "release" semantics.
https://en.cppreference.com/w/cpp/atomic/memory_order.html
> A store operation with this memory order performs the release operation: no reads or writes in the current thread can be reordered after this store.
> A store operation with this memory order performs the release operation: no reads or writes in the current thread can be reordered after this store. All writes in the current thread are visible in other threads that acquire the same atomic variable (see Release-Acquire ordering below) and writes that carry a dependency into the atomic variable become visible in other threads that consume the same atomic (see Release-Consume ordering below).
Relaxed atomic writes can be reordered in any way.
To quibble a little bit: later program-order writes CAN be reordered before release writes. But earlier program-order writes may not be reordered after release writes.
> Relaxed atomic writes can be reordered in any way.
To quibble a little bit: they can't be reordered with other operations on the same variable.
I stand corrected.
Regarding the style, it follows the "almost always auto" idea from Herb Sutter
if (next_head == buffer.size())
next_head = 0;
It's mentioned in the post, but worth reiterating!