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

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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>
   
 
Assuming:
  
Assuming:
−
* <code>x</code> is ''well-define'' (it contains no <code>unsafe...</code> calls),
 +
* <code>x</code> is ''well-defined'' (it contains no <code>unsafe...</code> calls),
 
* <code>x</code> is ''well-behaved'' (not throwing exceptions or causing errors);
  
* <code>x</code> is ''well-behaved'' (not throwing exceptions or causing errors);
   
Line 28: 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 concurrency, but with no visible side-effects - elementary parallelism.
 
|}
  
|}
   
−
* Now have a look at this <del>equally-as-ugly</del> prototype definition for <del><code>spawnIO</code></del> <code>forkIO</code>:
 +
* Now have a look at this <del>nearly-as-ugly</del> prototype definition for <del><code>spawnIO</code></del> <code>forkIO</code>:
 
:{|
  
:{|
 
|<haskell>
  
|<haskell>
 
forkIO :: IO () -> IO ThreadId
  
forkIO :: IO () -> IO ThreadId
−
forkIO act = do let t = unsafeLocalState 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 48: 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> is 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 is parallelism, but having visible side effects - it looks very much like elementary concurrency.
 
|}
  
|}
   
 
Can either of these prototypes ever go mainstream?
  
Can either of these prototypes ever go mainstream?
 
* As shown by it's type signature, <code>par</code> is supposed to be pure: avoiding the use of <code>unsafeLocalState</code> means making it primitive;
  
* As shown by it's type signature, <code>par</code> is supposed to be pure: avoiding the use of <code>unsafeLocalState</code> means making it primitive;
  +
* While the use of <code>unsafeInterleaveIO</code> may annoy some, it being one of the earlier Haskell extensions means it's widely available.
−
* Considering it's already <code>IO</code>-based, <code>forkIO</code> without <code>unsafeLocalState</code> seems more likely.
  
   
  +
For now, using a primitive <code>par</code> (and others) to define <code>forkIO</code> looks like the simplest option...but if using <code>unsafeInterleaveIO</code> ''really does'' annoy you, how about [[IO, partible-style|this]]:
−
[[IO, partible-style|This]] looks interesting:
  
  +
−
<haskell>
 +
:<haskell>
⚫
forkIO :: (OI -> ()) -> IO -> ThreadId
  
  +
forkIO :: (OI -> ()) -> OI -> ThreadId
 
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 20:58, 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 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:

  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 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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