libunifex VS Folly

I'm curious what led you to this conclusion. If you ran into scalability issues with its static_thread_pool, then that's a known issue. If it's something else, the authors (of which I'm one) would love to know.

Instead of calling add_executable you will call add_library. Here is a (only moderately complicated) production example of a library that can be built standalone (along with tests and example executables), or as a subproject, where it builds only the library

My original thought was to build my DDS implementation on top of libunifex in anticipation for standardization: https://github.com/facebookexperimental/libunifex

I'm trying to understand how to work with the proposed executors in a project, but after watching Eric Niebler's cppcon talks (https://youtu.be/xLboNIf7BTg) and looking at the libunifex examples (https://github.com/facebookexperimental/libunifex/tree/main/examples) I still have a hard time wrapping my head around how to employ the sender/receiver pattern in a larger project.

You have coroutines in C++20 but there is also the executives proposal that's making it's way into C++23 that is available as a library under the name unifex that only requires C++14

Asio-grpc makes exactly that possible by providing an Asio execution_context compatible interface to the CompletionQueue. It supports all types of RPCs (including generic ones), completion tokens, cancellation, as well as libunifex sender/receiver (if you want to try out what might become std::execution). The latest release (v1.7.0) also introduced a GrpcStream class for writing Rust/Golang select-style code.

P2300: they are trying to standardize facebookexperimental/libunifex

All the network handling for Instagram and all other Meta apps on all platforms is handled by their own C++ library https://github.com/facebookexperimental/libunifex.

This. I'd also like to mention libunifex. It's entirely based on tag_invoke and is a testament as to how much power it actually provides. On the other hand, it also proves how cumbersome it is to define CPOs with tag_invoke. But IMO it's a lot better than anything else anyone has ever created, and users usually don't need to define new CPOs, only library writers do, so there's that.

A work in progress implementation of the library: https://github.com/facebookexperimental/libunifex

https://github.com/facebook/folly/commit/b1391e1c57be71c1e2a...

https://github.com/facebook/folly/pull/2153

To set a HP on Linux, Folly just does a relaxed load of the src pointer, release store of the HP, compiler-only barrier, and acquire load. (This prevents the compiler from reordering the 2nd load before the store, right? But to my understanding does not prevent a hypothetical CPU reordering of the 2nd load before the store, which seems potentially problematic!)

Then on the GC/reclaim side of things, after protected object pointers are stored, it does a more expensive barrier[0] before acquire-loading the HPs.

I'll admit, I am not confident I understand why this works. I mean, even on x86, loads can be reordered before earlier program-order stores. So it seems like the 2nd check on the protection side could be ineffective. (The non-Linux portable version just uses an atomic_thread_fence SeqCst on both sides, which seems more obviously correct.) And if they don't need the 2nd load on Linux, I'm unclear on why they do it.

[0]: https://github.com/facebook/folly/blob/main/folly/synchroniz...

(This uses either mprotect to force a TLB flush in process-relevant CPUs, or the newer Linux membarrier syscall if available.)

folly provides functions to resize std::string & std::vector without initialization [0].

[0] https://github.com/facebook/folly/blob/3c8829785e3ce86cb821c...

My implementation was quite similar to Boost's spsc_queue and Facebook's folly/ProducerConsumerQueue.h.

> How is that relevant?

Roaring bitmaps and similar data structures get their speed from decoding together consecutive groups of elements, so if you do sequential decoding or decode a large fraction of the list you get excellent performance.

EF instead excels at random skipping, so if you visit a small fraction of the list you generally get better performance. This is why it works so well for inverted indexes, as generally the queries are very selective (otherwise why do you need an index?) and if you have good intersection algorithms you can skip a large fraction of documents.

I didn't follow the rest of your comment, select is what EF is good at, every other data structure needs a lot more scanning once you land on the right chunk. With BMI2 you can also use the PDEP instruction to accelerate the final select on a 64-bit block: https://github.com/facebook/folly/blob/main/folly/experiment...

C++ with folly's SCOPE_EXIT {} construct:

https://github.com/facebook/folly/blob/main/folly/ScopeGuard...

Seems like github issues taking a long time to get any response: https://github.com/facebook/folly

Can't speak for abseil and tbb, but in folly there are a few solutions for the common problem of sharing state between a writer that updates it very infrequently and concurrent readers that read it very frequently (typical use case is configs).

The most performant solutions are RCU (https://github.com/facebook/folly/blob/main/folly/synchroniz...) and hazard pointers (https://github.com/facebook/folly/blob/main/folly/synchroniz...), but they're not quite as easy to use as a shared_ptr [1].

Then there is simil-shared_ptr implemented with thread-local counters (https://github.com/facebook/folly/blob/main/folly/experiment...).

If you absolutely need a std::shared_ptr (which can be the case if you're working with pre-existing interfaces) there is CoreCachedSharedPtr (https://github.com/facebook/folly/blob/main/folly/concurrenc...), which uses an aliasing trick to transparently maintain per-core reference counts, and scales linearly, but it works only when acquiring the shared_ptr, any subsequent copies of that would still cause contention if passed around in threads.

[1] Google has a proposal to make a smart pointer based on RCU/hazptr, but I'm not a fan of it because generally RCU/hazptr guards need to be released in the same thread that acquired them, and hiding them in a freely movable object looks like a recipe for disaster to me, especially if paired with coroutines https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p05...

Not sure if it's still the case but about 6 years ago Facebook's folly C++ library was something I'd point to for my junior engineers to get a sense of "good" C++ https://github.com/facebook/folly