#4167: Add numSparks# to return the number of elements in the current
capability's
spark queue
---------------------------------+------------------------------------------
Reporter: ekmett | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.12.3 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
Adding
{{{
numSparks# :: State# s -> (# State# s, Int# #)
}}}
returning
{{{
dequeElements(cap->spark)
}}}
from the c-- backend would permit user code in Haskell to inspect the
current capability's spark backlog to decide whether or not to spark a
task at all.
Consider how the following combinator from 'speculation' works. Under
load, the par will cause the speculation to be discarded, but otherwise it
can use it to begin evaluating the function before the argument a is
ready, enhancing parallelism if the guess is accurate.
{{{
spec :: Eq a => a -> (a -> b) -> a -> b
spec guess f a =
speculation `par`
if guess == a
then speculation
else f a
where
speculation = f guess
Under high load:
foreground: [----- a -----]
foreground: [-] (check g == a)
foreground: [---- f a ----]
overall: [-------- spec g f a ---------]
}}}
However, a similar combinator needs must degrade much worse under load.
{{{
specSTM :: Eq a => a -> (a -> STM b) -> a -> STM b
}}}
Due to the fact that the computation of the function is an `STM`
calculation ''that needs access to the current transaction'', it can't be
moved into the spark. However, it *could* inspect `numSparks#` to
determine if the current capability's spark queue was loaded and therefore
would backlog anyways to avoid evaluating `f g` at all under load,
allowing it to (somewhat clumsily) ape the graceful degradation under load
experienced by `spec`.
This unfortunately cannot be done directly in a third party package using
`foreign import prim` because the capability and spark queue are declared
private and so GHC's codebase is the only place that has access to the
appropriate primitives.
For more timelines and examples, see 'speculation'.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4167>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
