On 07/11/2011 14:50, Ryan Newton wrote:
Hi GHC users,
When implementing certain concurrent systems-level software in Haskell
it is good to be aware of all potentially blocking operations.
Presently, blocking on an MVar is explicit (it only happens when you
do a takeMVar), but blocking on a BLACKHOLE is implicit and can
potentially happen anywhere.
If there are known thunks where we, the programmers, know that
contention might occur, would it be possible to create a variant of
"Control.Monad.Evaluate" that allows us to construct non-blocking
software:
evaluate :: a -> IO a
evaluateNonblocking :: a -> IO (Maybe a)
It would simply return Nothing if the value is BLACKHOLE'd. Of course
it may be helpful to also distinguish the evaluated and unevaluated
states. Further, the above simple version allows data-races (it may
become blackhole'd right after we evaluate). An extreme version would
actively blackhole it to "lock" the thunk... but maybe that's overkill
and there are some other good ideas out there.
A mechanism like the proposed should, for example, allow us to consume
just as much of a lazy Bytestring as has already been computed by a
producer, WITHOUT blocking and waiting on that producer thread, or
migrating the producer computation over to our own thread (blowing its
cache).
The problem is that a thunk may depend on other thunks, which may or may
not themselves be BLACKHOLEs. So you might be a long way into
evaluating the argument and have accumulated a deep stack before you
encounter the BLACKHOLE.
Hmm, but there is something you could do. Suppose a thread could be in
a mode in which instead of blocking on a BLACKHOLE it would just throw
an asynchronous exception WouldBlock. Any computation in progress would
be safely abandoned via the usual asynchronous exception mechanism, and
you could catch the exception to implement your evaluateNonBlocking
operation.
I'm not sure this would actually be useful in practice, but it's
certainly doable.
Cheers,
Simon
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