by u1tron on 1/7/22, 4:41 PM with 83 comments
by ot on 1/7/22, 8:00 PM
folly is a community effort within Meta; it doesn't have a specific theme about what's included in it, but it is a set of core libraries that are likely to be depended on by most C++ software within the company, and are enough high-quality and self-contained that they can be useful externally, or showcase implementations undergoing standardization efforts (for example experimental/coro for coroutines).
Main focus is server applications, but it is also used in mobile, in particular Thrift (the serialization/RPC infrastructure) depends on it.
A very personal list of favorites:
- experimental/coro is the coroutines infrastructure, which is now widely adopted.
- SharedMutex is a heavily optimized mutex which supports shared/upgrade/exclusive semantics, and allows near-linear scalability in shared mode. It is used pretty much everywhere as it is the default mutex for Synchronized, a wrapper to safely synchronize access to objects. Its footprint is just 4 bytes.
- DistributedMutex, OTOH, is an exclusive-only mutex that supports flat combining, and performs extremely well under high contention.
- MPMCQueue and UnboundedQueue are lock-free MPMC queues, respectively bounded and unbounded, which are used in almost all our thread pools (see the executor/ directory)
- Userland implementations of RCU and hazard pointers, for deferred reclamation.
- LifoSem and Baton are other fundamental synchronization primitives that are not available in most core C++ libraries. They do a magic trick internally, where if they're waiting long enough, they unmap the unused suffix of the stack, so idle threads don't hold unused stack memory.
- Function is like std::function, but move-only, so it can hold non-copyable function objects (think lambda that captures unique_ptr). It is the vocabulary type for callbacks used in futures and executors.
- F14 is a SIMD-optimized hash table. Similar to Abseil's SwissTable, it was published around the same time.
- TDigest is a highly scalable quantile estimator, widely used for service counters (see fb303, also open source).
- (shameless plug, as I wrote this one) enumerate() is a replica of the Python function, so you can do
for (auto&& [i, element] : folly::enumerate(collection)) { ... }
by dfghdfhs on 1/7/22, 6:16 PM
Right now the title reads essentially: "Here's a library".
by interroboink on 1/7/22, 9:21 PM
However, I did have decent success taking the classes I was interested in (CPUThreadPoolExecutor, and surrounding), stripping the library down to just those bits, and using them. Obviously a PITA to stay current with upstream, though.
If nothing else, it's worth reading for some of the highly-optimized stuff going on in there.
by ComputerGuru on 1/7/22, 5:48 PM
by zoomablemind on 1/7/22, 8:29 PM
I understand the FB rationale to have it, but what whould be a reason for other projects to adopt this lib?
I won't imagine maintaining such a lib as a dependency by ourselves. Maybe for one short-term purpose, but then it's still safer to go with standard or boost, or, God forbid, carve out some pieces and wrap them up.
by alcover on 1/7/22, 6:25 PM
More precisely, one that defines a datatype (no SDS-like trick) and has all goodies like SSO, CoW, views, thread-safety, etc..
I'm making such a lib but can't seem to find other examples to compare.
by derefnull on 1/8/22, 11:56 PM
folly [https://www.wordnik.com/words/folly]
fŏl′ē
noun
Lack of good sense, understanding, or foresight.
An act or instance of foolishness.
A costly undertaking having an absurd or ruinous outcome.
“acronymed loosely after Facebook Open Source Library”
by didip on 1/7/22, 10:21 PM
by lumost on 1/7/22, 10:32 PM
by vorhemus on 1/7/22, 8:22 PM
by cjensen on 1/7/22, 6:30 PM
by seertaak on 1/7/22, 7:53 PM