Cheng Shao authored 1 year ago and Marge Bot committed 1 year ago

hs_try_putmvar002 includes pthread.h and doesn't work on targets
without this header (e.g. wasm32). It doesn't need to include this
header at all. This was previously unnoticed by wasm CI, though recent
toolchain upgrade brought in upstream changes that completely removes
pthread.h in the single-threaded wasm32-wasi sysroot, therefore we
need to handle that change.

Summary:
This is a fast, non-blocking, asynchronous, interface to tryPutMVar that
can be called from C/C++.

It's useful for callback-based C/C++ APIs: the idea is that the callback
invokes hs_try_putmvar(), and the Haskell code waits for the callback to
run by blocking in takeMVar.

The callback doesn't block - this is often a requirement of
callback-based APIs.  The callback wakes up the Haskell thread with
minimal overhead and no unnecessary context-switches.

There are a couple of benchmarks in
testsuite/tests/concurrent/should_run.  Some example results comparing
hs_try_putmvar() with using a standard foreign export:

    ./hs_try_putmvar003 1 64 16 100 +RTS -s -N4     0.49s
    ./hs_try_putmvar003 2 64 16 100 +RTS -s -N4     2.30s

hs_try_putmvar() is 4x faster for this workload (see the source for
hs_try_putmvar003.hs for details of the workload).

An alternative solution is to use the IO Manager for this.  We've tried
it, but there are problems with that approach:
* Need to create a new file descriptor for each callback
* The IO Manger thread(s) become a bottleneck
* More potential for things to go wrong, e.g. throwing an exception in
  an IO Manager callback kills the IO Manager thread.

Test Plan: validate; new unit tests

Reviewers: niteria, erikd, ezyang, bgamari, austin, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2501