Folly VS serenity

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

SerenityOS replicates that look and feel. It is also implemented in a dialect of C++ that adheres to some of the good parts of C++98: https://serenityos.org

One example of a useful technique

https://serenityos.org/ apparently only makes source code available. There are no binary images of the OS to install

I think Andreas said this functions like a little test -- if you're not willing to build it from source, then you probably wouldn't be a good contributor anyway.

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Likewise, my shell project provides source tarballs only, right now - https://www.oilshell.org/release/0.21.0/

It is packaged in a number of places, which I appreciate. That means some other people are willing to do some work.

And they provide good feedback.

I would like it to be more widely available, but yeah I definitely see that you need to "gate" peanut gallery feedback a bit, because it takes up a lot of time.

Of course, it's a tricky balance, because you also want feedback from casual users, to make the project better.

Indeed, given the existence of `JS::NonnullGCPtr`, `JS::GcPtr` intentionally corresponds to a nullable pointer, so it seems dangerous to convert one to a reference without a null-check.

That said, a naive code search finds what *may* be more cases of this pattern:

https://github.com/search?q=repo%3ASerenityOS%2Fserenity+%2F...

Eg: https://github.com/SerenityOS/serenity/blob/a68b134e6dea5065... -> https://github.com/SerenityOS/serenity/blob/a68b134e6dea5065...

In some of those search results, it is fine because there is a preceding null-check, and obviously I know nothing about this code other than this naive search result, but perhaps it would be prudent to vet all of them.

It is a SerenityOS project. You can find the answer to that question in their primary project's FAQ[1].

1. https://github.com/SerenityOS/serenity/blob/master/Documenta...

https://github.com/SerenityOS/serenity

The best way to write proper exception free C++ is not to use the C++ Standard Library.

I initially confused it with Serenity OS prior to watching the video: https://github.com/SerenityOS/serenity

My contributions to SerenityOS[0] helped me get my current job. My team lead (who was also my interviewer) was interested in what I did since I listed some of it in my CV, and I showed him some PRs I made and explained what went into each of them. It was really exciting because I didn't have professional experience with low-level development, and basically got the job due to hobby programming.

[0]: https://github.com/SerenityOS/serenity/pulls?q=is%3Apr+autho...

Definitely not "literally impossible", just a great deal of work. https://github.com/SerenityOS/serenity/tree/master/Ladybird