• Simon Marlow's avatar
    Add hs_try_putmvar() · 454033b5
    Simon Marlow authored
    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
    454033b5
hs_try_putmvar003.hs 2.54 KB