Talk:Parallelism vs. Concurrency

Parallelism vs concurrency: what's the difference?

Visible side effects.

• Have a look at this ugly eysore "prototype definition" of par:

par :: a -> b -> b par x y = case unsafeLocalState (forkIO (evaluate x >> return ())) of !_ -> y where: evaluate :: a -> IO a forkIO :: IO () -> IO ThreadId Assuming: x is well-defined (it contains no unsafe... calls), x is well-behaved (not throwing exceptions or causing errors); then: forkIO attaches a ThreadId to its argument, adds it to the work-queue and returns the identifier; par then returns y; Some time later, forkIO's argument is called, causing evaluate to start evaluating x. If y is still being evaluated when the evaluation of x commences, then we have concurrency, but with no visible side-effects - elementary parallelism.

• Now have a look at this nearly-as-ugly prototype definition for spawnIO forkIO:

forkIO :: IO () -> IO ThreadId forkIO act = do t <- unsafeInterleaveIO act case attachThreadId t of Nothing -> itsThreadId t Just i -> par t (return i) where: attachThreadId :: a -> IO (Maybe ThreadId) itsThreadId :: a -> IO ThreadId Assuming: par, attachThreadId and itsThreadId are primitive, attachThreadId would return Nothing if it's argument already has been assigned a ThreadId; then: A new ThreadId is assigned to the new (and suspended) value t; evaluating par t i causes t to be added to the work-queue by the implementation; the ThreadId for t is then returned; Some time later, the implementation discovers that a ThreadId has been attached to t and uses the ThreadId to immediately start a new thread for evaluating t; This is parallelism, but having visible side effects - it looks very much like elementary concurrency.

Can either of these prototypes ever go mainstream?

• As shown by it's type signature, par is supposed to be pure: avoiding the use of unsafeLocalState means making it primitive;
• While the use of unsafeInterleaveIO may annoy some, it being one of the earlier Haskell extensions means it's widely available.

For now, using a primitive par (and others) to define forkIO looks like the simplest option...but if using unsafeInterleaveIO really does annoy you, how about this:

forkIO       :: (OI -> ()) -> OI -> ThreadId
forkIO act u =  let !(u1:u2:u3:_) = parts u in
                let t             = act u1 in
                case attachThreadId t u2 of
                  Nothing -> itsThreadId t u3
                  Just i  -> par t i

-- Atravers Tue Apr 20 06:04:10 UTC 2021

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