Parallelism!

[byorgey]

diagrams-haddock has some embarrassing parallelism going on. We should add a -j flag that turns on multithreading.

[byorgey]

Some notes:

• spawn (http://hackage.haskell.org/package/spawn-0.3) has an abstraction for running things in a pool but it uses a deprecated API.
• Can instead just use a QSem, something like newQSem n >>= \qs -> async (map (bracket (waitQSem qs) (signalQSem qs)) actions) (from @luite)
• Or use a queue of tasks and fork off n workers with async. Can't use Chan since there's no way to check if it's empty. Instead use Control.Concurrent.STM.TChan.
• From @fryguybob: worker c = join $ atomically (readTChan c >>= return . (>> worker c) <|> return ())

[fryguybob]

I implemented something on the parallel branch 1d5bc08 but when I run I get the disappointing message:

This version of GHC is not thread-safe,can't safely run two instances of the interpreter simultaneously

How sad!

[byorgey]

Bummer! See http://ghc.haskell.org/trac/ghc/ticket/3373 . I suppose one workaround would be to actually fire up N copies of the RTS in different OS processes? But that would probably have a lot of overhead, and I don't know if it would be worth the trouble.