Folly Alternatives

Folly reviews and mentions

• Ask HN: How bad is the xz hack?
  1 project | news.ycombinator.com | 31 Mar 2024

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

• Backdoor in upstream xz/liblzma leading to SSH server compromise
  49 projects | news.ycombinator.com | 29 Mar 2024

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

• A lock-free ring-buffer with contiguous reservations (2019)
  9 projects | news.ycombinator.com | 29 Feb 2024

  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.)

• Appending to an std:string character-by-character: how does the capacity grow?
  2 projects | news.ycombinator.com | 26 Oct 2023

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

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

• Can anyone explain feedback of a HFT firm regarding implementation of SPSC lock-free ring-buffer queue?
  1 project | /r/highfreqtrading | 12 Jul 2023

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

• A Compressed Indexable Bitset
  6 projects | news.ycombinator.com | 1 Jul 2023

  > 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...

• Defer for Shell
  1 project | news.ycombinator.com | 20 Jun 2023

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

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

• Is there any facebook/folly community for discussion and Q&A?
  1 project | /r/cpp | 19 Jun 2023

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

• How a Single Line of Code Made a 24-Core Server Slower Than a Laptop
  4 projects | news.ycombinator.com | 17 Jun 2023

  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...

• Ask HN: What are some of the most elegant codebases in your favorite language?
  37 projects | news.ycombinator.com | 17 Jun 2023

  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

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