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

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m (GHC provides "evaluate", not "evalIO"...)
(Selected code examples modified)
Line 37: Line 37:
 
forkIO :: IO () -> IO ThreadId
 
forkIO :: IO () -> IO ThreadId
 
forkIO act = do t <- unsafeInterleaveIO act
 
forkIO act = do t <- unsafeInterleaveIO act
case par t () of
+
case attachThreadId t of
!_ -> do i' <- itsThreadId t
+
Nothing -> itsThreadId t
case i' of
+
Just i -> par t (return i)
Just i -> return i
 
Nothing -> ioError "forkIO"
 
 
</haskell>
 
</haskell>
   
Line 45: Line 45:
   
 
<haskell>
 
<haskell>
itsThreadId :: a -> IO (Maybe ThreadId)
+
attachThreadId :: a -> IO (Maybe ThreadId)
  +
itsThreadId :: a -> IO ThreadId
 
</haskell>
 
</haskell>
   
 
Assuming:
 
Assuming:
* <code>par</code> and <code>itsThreadId</code> are primitive,
+
* <code>par</code>, <code>attachThreadId</code> and <code>itsThreadId</code> are primitive,
* <code>itsThreadId</code> would return <code>Nothing</code> if it's argument had not been previously used by <code>par</code>;
+
* <code>attachThreadId</code> would return <code>Nothing</code> if it's argument already has been assigned a <code>ThreadId</code>;
   
 
then:
 
then:
# Evaluating <code>par t ()</code> causes a new <code>ThreadId</code> to be attached to <code>t</code> by the implementation;
+
# A new <code>ThreadId</code> is assigned to the new (and suspended) value <code>t</code>;
# <code>itsThreadId</code> retrieves <code>i'</code>, the (possible) identifier for <code>t</code>;
+
# evaluating <code>par t i</code> causes <code>t</code> to be added to the work-queue by the implementation;
# <code>forkIO</code> then extracts and returns the identifier.
+
# the <code>ThreadId</code> for <code>t</code> is then returned;
  +
# Some time later, the implementation discovers that a <code>ThreadId</code> has been attached to <code>t</code> and uses the <code>ThreadId</code> to immediately start a new thread for evaluating <code>t</code>;
   
 
This looks very much like elementary concurrency: parallelism, but having visible side effects.
 
This looks very much like elementary concurrency: parallelism, but having visible side effects.
Line 70: Line 70:
 
forkIO act u = let !(u1:u2:u3:_) = parts u in
 
forkIO act u = let !(u1:u2:u3:_) = parts u in
 
let t = act u1 in
 
let t = act u1 in
case par t () of
+
case attachThreadId t u2 of
!_ -> case itsThreadId t u2 of
+
Nothing -> itsThreadId t u3
Just i -> i
+
Just i -> par t i
Nothing -> ioError "forkIO" u3
 
 
</haskell>
 
</haskell>
   

Revision as of 20:50, 14 May 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 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:

  1. A new ThreadId is assigned to the new (and suspended) value t;
  2. evaluating par t i causes t to be added to the work-queue by the implementation;
  3. the ThreadId for t is then returned;
  4. 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 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 attachThreadId t u2 of
                  Nothing -> itsThreadId t u3
                  Just i  -> par t i

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