Difference between revisions of "Talk:Parallelism vs. Concurrency"

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(Using unsafeInterleaveIO where applicable)
m (GHC provides "evaluate", not "evalIO"...)
 
Line 8: Line 8:
 
par :: a -> b -> b
 
par :: a -> b -> b
 
par x y = case
 
par x y = case
unsafeLocalState (forkIO (evalIO x >> return ()))
+
unsafeLocalState (forkIO (evaluate x >> return ()))
 
of
 
of
 
!_ -> y
 
!_ -> y
Line 16: Line 16:
   
 
<haskell>
 
<haskell>
evalIO :: a -> IO a
+
evaluate :: a -> IO a
forkIO :: IO () -> IO ThreadId
+
forkIO :: IO () -> IO ThreadId
 
</haskell>
 
</haskell>
   
Line 27: Line 27:
 
# <code>forkIO</code> attaches a <code>ThreadId</code> to its argument, adds it to the work-queue and returns the identifier;
 
# <code>forkIO</code> attaches a <code>ThreadId</code> to its argument, adds it to the work-queue and returns the identifier;
 
# <code>par</code> then returns <code>y</code>;
 
# <code>par</code> then returns <code>y</code>;
# Some time later, <code>forkIO</code>'s argument is called, causing <code>evalIO</code> to start evaluating <code>x</code>.
+
# Some time later, <code>forkIO</code>'s argument is called, causing <code>evaluate</code> to start evaluating <code>x</code>.
   
 
If <code>y</code> is still being evaluated when the evaluation of <code>x</code> commences, then we have elementary parallelism: concurrency, but with no visible side-effects.
 
If <code>y</code> is still being evaluated when the evaluation of <code>x</code> commences, then we have elementary parallelism: concurrency, but with no visible side-effects.

Latest revision as of 05:25, 21 April 2021

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:

  1. forkIO attaches a ThreadId to its argument, adds it to the work-queue and returns the identifier;
  2. par then returns y;
  3. 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 elementary parallelism: concurrency, but with no visible side-effects.

  • 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 par t () of
                   !_ -> do i' <- itsThreadId t
                            case i' of
                              Just i  -> return i
                              Nothing -> ioError "forkIO"

where:

      itsThreadId :: a -> IO (Maybe ThreadId)

Assuming:

  • par and itsThreadId are primitive,
  • itsThreadId would return Nothing if it's argument had not been previously used by par;

then:

  1. Evaluating par t () causes a new ThreadId to be attached to t by the implementation;
  2. itsThreadId retrieves i', the (possible) identifier for t;
  3. forkIO then extracts and returns the identifier.

This looks very much like elementary concurrency: parallelism, but having visible side effects.

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 par t () of
                  !_ -> case itsThreadId t u2 of
                          Just i  -> i
                          Nothing -> ioError "forkIO" u3

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