Add hs_try_putmvar()

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

1 files changed, 1 insertions, 0 deletions

diff --git a/rts/Capability.c b/rts/Capability.c
index 681797b409..6979c637bc 100644
--- a/rts/Capability.c
+++ b/rts/Capability.c
@@ -266,6 +266,7 @@ initCapability (Capability *cap, uint32_t i)
     cap->returning_tasks_tl = NULL;
     cap->n_returning_tasks  = 0;
     cap->inbox              = (Message*)END_TSO_QUEUE;
+    cap->putMVars           = NULL;
     cap->sparks             = allocSparkPool();
     cap->spark_stats.created    = 0;
     cap->spark_stats.dud        = 0;