> NOTE: Throughout our implementation we strictly use compare_exchange_strong, but the C++ standard suggests that for some systems like ARM it mighe be a better idea to run a loop with compare_exchange_weak for better performance. The problem with that is a compare_exchange_weak call may fail spuriously, which essentially it can randomly fail even if everything is correct, thus it makes code code a bit more complicated, so I avoided it for this implementation.
The reason using `compare_exchange_weak` is a better idea for lock-free algorithms is that, in most cases, you'll run it in a retry loop anyway. Since `compare_exchange_strong` is compiled to a retry loop, if you do a retry loop of `compare_exchange_strong` you basically have a loop in a loop. Using `compare_exchange_weak` makes things both simpler and more performant.
This is overstated in value; on arm systems with LSE it's faster to use that even for weak operations than to use ll/sc. Even if you are limited to ll/sc the compiler may not put your cas-loop body into the ll/sc region as there's limitations on how many and what type of instructions are permitted there.
Sure, with LSE. But you have to target LSE and get a system that actually contains it, i.e. not the default compiler flags and not the original AWS Graviton.
Not trying to be critical, but there are a number of misspellings and grammatical issues and it was actually a breath of fresh air to be reminded while I was reading that a real human being wrote this. I feel a little inspired to turn off spell check for my own writing.
Nice article. There a few issues with your code however from a cursory glance; your dtor seems to allow for spurious/double frees due to custom deleter support (you wanna check up on that), you also seem to use seq_cst far too much even if not needed (you want to avoid them is queues as much as possible), lastly class FastQueueNodeSlot.. isn't aligned (plus 64b alignment is only a thing for amd64 cpus, apple silicon is larger).
a C++ experienced programmer, I spoke recently to, told me that using new and delete is basically prohibited nowadays in C++ in favour of std::make_unique, std::make_shared etc.
For 95% of all code that is correct. std::make_* is easy to use and prevents a lot of mistakes, while having no loss of performance. That last 5% though you are doing weird things and so need to do something manually. (as the other poster said, make_unique is implemented with new) Of course the 5% is overall. Some projects never have anything that gets into that last 5%, while others it is more like 50% of the code can't use make_*. If at all possible your use of new/delete should be limited to a data structure/container than handles it, and not scatters all over.
"For 95% of all code that is correct. std::make_* is easy to use and prevents a lot of mistakes, while having no loss of performance."
For 95% of code std::unique_ptr has no loss of performance. Perhaps. Just remember that it is not a zero-cost abstraction, the compiler won't always be able to entirely eliminate the overhead:
I watched that about 7 years ago when it first came out, and at an hour long I'm not about to watch it again... From what I can recall those are things that rarely are important. There is a cost, but in most real world code they only add a few nanoseconds - I have more important things to worry about.
Even at 32Mhz you typically have more important things to worry about. These nanoseconds are unlikely to add up - the overhead from the underlying new/delete is going to be much worse.
Classic C++, any issue is at most three years away from going away! There will definitely not be any other issues that are also three years from getting fixed when you reach that year though
I don't think adding unsafe will be possible - it will break too much existing code. We might be able to add something new that is only possible in an unsafe context, but there is too much existing code.
However I do expect a [[safe]] profile (or perhaps several, depending on which paper you read) that everyone is encourage to opt-in to. Likely combines with compiler warnings and static analysis to encourage that use. (Also syntax is still open for debate)
I was for Safe C++ paper, based on Circle experience, but it was shot down due to politics.
So we're left with the profiles camp actually delivering, followed by the remaining compilers caring to actually implement them, otherwise it will be static and dynamic analysis as usual.
Yeah, that is why there was a paper created specifically to kill any other proposal, by the profiles folks with WG21 majority, and ironically profiles are just as annotation heavy, but since it is profiles, it is alright.
I call that politics, and in the end the most likely outcome is that by C++29 nothing will be delivered by the profiles group, that is any better than using clang-tidy already today.
Agreed. But you make it sound like the worst case is necessarily fatal. It depends on the use-case. The workable cmpxchg algorithms will make progress on at least one core each round. In a push or pop operation one of the cmpxchg must have succeeded for another to fail. The atomic xchg algorithms that I know of have other undesirable pathologies (e.g. a suspended producer can stall the consumer).
I'd ask your friendly local LLM for design advice, but off the top of my head, so long as `static_assert(std::atomic<whatever>::is_lock_free)` passes, you could build a lock-free SPSC queue with just plain ol' std::atomics and probably liberal use of `alignas()` calls
Most local LLMs I've used are not particularly great with the advice unfortunately, probably because I haven't spent thousands of dollars on new hardware recently
Whether the allocator calls are "naive" or not depends entirely on the allocator in use. If you need thread/core locality and batching you can get that by replacing global new/delete functions with a decent allocator.
The reason using `compare_exchange_weak` is a better idea for lock-free algorithms is that, in most cases, you'll run it in a retry loop anyway. Since `compare_exchange_strong` is compiled to a retry loop, if you do a retry loop of `compare_exchange_strong` you basically have a loop in a loop. Using `compare_exchange_weak` makes things both simpler and more performant.
For 95% of code std::unique_ptr has no loss of performance. Perhaps. Just remember that it is not a zero-cost abstraction, the compiler won't always be able to entirely eliminate the overhead:
https://www.youtube.com/watch?v=rHIkrotSwcc
However I do expect a [[safe]] profile (or perhaps several, depending on which paper you read) that everyone is encourage to opt-in to. Likely combines with compiler warnings and static analysis to encourage that use. (Also syntax is still open for debate)
I was for Safe C++ paper, based on Circle experience, but it was shot down due to politics.
So we're left with the profiles camp actually delivering, followed by the remaining compilers caring to actually implement them, otherwise it will be static and dynamic analysis as usual.
I call that politics, and in the end the most likely outcome is that by C++29 nothing will be delivered by the profiles group, that is any better than using clang-tidy already today.
The last thing you want is all of the threads failing to cmpxchg (spuriously or otherwise ) spinning on a shared cacheline
Real world alternatives show atomic xchg only solutions scale to hundreds of threads.
But notably only with certain workloads