Building a Fast Lock-Free Queue in Modern C++ from Scratch

rfgplk•about 2 hours ago

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).

p0w3n3d•35 minutes ago

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.

bluGill•22 minutes ago

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.

RossBencina•16 minutes ago

"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:

https://www.youtube.com/watch?v=rHIkrotSwcc

RossBencina•29 minutes ago

In many application code bases no doubt. But how do you think make_unique and make_shared are implemented?

moffers•about 1 hour ago

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.

nly•about 1 hour ago

Once you use atomic cmpxchg you've lost a great deal of scalability because it implies a retry loop (internal or by the user)

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.

RossBencina•21 minutes ago

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).

adzm•about 1 hour ago

> Real world alternatives show atomic xchg only solutions scale to hundreds of threads

But notably only with certain workloads

nly•about 1 hour ago

Once you get to using custom lock free queues you should be picking something that matches your workload/broader design anyway.

PcChip•about 1 hour ago

Is this similar to moodycamel’s?

Building a Fast Lock-Free Queue in Modern C++ from Scratch

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