July 2018 - ghc-tickets

[GHC] #14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified
by GHC 13 Sep '18

13 Sep '18

#14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This came up on a [https://www.reddit.com/r/haskell/comments/8257mz/how_quantifiedconstraints_… reddit thread], {{{#!hs {-# Language QuantifiedConstraints #-} class (forall aa. Functor (bi aa)) => Bifunctor bi where first :: (a -> a') -> (bi a b -> bi a' b) bimap :: Bifunctor bi => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') bimap f g = first f . fmap g }}} This is the type we want for `bimap` even if we mix & match `Bifunctor` and `Functor`. We already know that we can `fmap @(bi xx)` for any `xx` but this is not the inferred type. Instead GHC infers a type (tidied up) with a superfluous `Functor` constraint {{{#!hs bimap :: (Bifunctor bi, Functor (bi a)) => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') }}} Indeed post-composing with a superfluous `fmap @(bi a')` incurs yet another `Functor` constraint {{{#!hs bimap :: (Bifunctor bi, Functor (bi a), Functor (bi a')) => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') bimap f g = fmap id . first f . fmap g }}} A terminology question, I'm not sure how to phrase what GHC isn't doing to the `Functor` constraints: ‘discharge’? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14896> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15285: "strange closure type" in T7919 with the threaded2 way
by GHC 13 Sep '18

13 Sep '18

#15285: "strange closure type" in T7919 with the threaded2 way ----------------------------------------+---------------------------------- Reporter: alpmestan | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 8.5 Keywords: | Operating System: Linux Architecture: Unknown/Multiple | Type of failure: Runtime crash Test Case: T7919 | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+---------------------------------- Last night's `./validate --slow` run on Circle CI revealed the following failure: {{{ Wrong exit code for T7919(threaded2)(expected 0 , actual 134 ) Stderr ( T7919 ): T7919: internal error: evacuate: strange closure type 32554624 (GHC version 8.5.20180617 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Aborted *** unexpected failure for T7919(threaded2) }}} The full log is available [https://circleci.com/api/v1.1/project/github/ghc/ghc/6205/output/107/0?file… here]. Among other things, you can see that it indeed only fails in the threaded2 way. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15285> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15359: Quantified constraints do not work with equality constraints
by GHC 13 Sep '18

13 Sep '18

#15359: Quantified constraints do not work with equality constraints -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Feeling abusive toward GHC this morning, I tried this: {{{#!hs class C a b data Dict c where Dict :: c => Dict c foo :: (forall a b. C a b => a ~ b) => Dict (C a b) -> a -> b foo Dict x = x }}} I thought it wouldn't work, and I was right: {{{ • Class ‘~’ does not support user-specified instances • In the quantified constraint ‘forall (a :: k) (b :: k). C a b => a ~ b’ In the type signature: foo :: (forall a b. C a b => a ~ b) => Dict (C a b) -> a -> b }}} Good. But then I tried something more devious: {{{#!hs class C a b class a ~ b => D a b data Dict c where Dict :: c => Dict c foo :: (forall a b. C a b => D a b) => Dict (C a b) -> a -> b foo Dict x = x }}} This also fails, with ` Couldn't match expected type ‘b’ with actual type ‘a’`. I'm fine with the second case failing (because I think anything else would be very challenging), but the error message is unfortunate. According to the semantics of class constraints and quantified constraints, this case should be accepted. So if we reject it, we should have a good reason -- like we offer in the first case. Essentially, we need to explain that any logical entailment of an equality constraint simply doesn't hold in the presence of a quantified constraint. I neither know a good pithy way of explaining that to users nor how to detect when to do so. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15359> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14025: Object file is put in wrong directory when any source has absolute path
by GHC 13 Sep '18

13 Sep '18

#14025: Object file is put in wrong directory when any source has absolute path -------------------------------------+------------------------------------- Reporter: literon | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Files: `/tmp/e/A.hs`, `/tmp/e/dep.c`. Current dir: `/tmp/e`. `ghc -odir /tmp/xyzzy A.hs dep.c`: works as expected, puts `dep.o` in `/tmp/xyzzy`. But try to prefix either `A.hs` or `dep.c` (or both) with `/tmp/e/`, and suddenly `dep.o` is put in `/tmp/e`, not in `/tmp/xyzzy`. Couldn't check with 8.2.1 yet. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14025> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11197: Overeager deferred type errors
by GHC 11 Sep '18

11 Sep '18

#11197: Overeager deferred type errors -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- With `TypeInType`, the solver now works in terms of unlifted equality. The only place this matters is in deferred type errors, which now are more eager than they were before. For example: {{{ {-# OPTIONS_GHC -fdefer-type-errors #-} module Main where main = do putStrLn "Hi there." putStrLn True }}} This used to print {{{ Hi there. Defer: Defer.hs:7:12: error: ... }}} Now it prints {{{ Defer: Defer.hs:7:12: error: ... }}} No more `Hi there.`. Thinking about Core, this change should be expected. GHC puts all evidence bindings for `main` at the top, and with unlifted equality in the solver, these bindings now contain `case typeError ... of ...`, which forces the type error eagerly. Previously, there was a lazy binding for a lifted equality witness, unpacked only at the last moment by magic in the desugarer, thus the lazier warning. Simon has proposed customizing the !FloatIn pass to deal with this scenario. I do believe this would work nicely, but I have not yet implemented it. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11197> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12833: GHCi
by GHC 10 Sep '18

10 Sep '18

#12833: GHCi -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In multiline mode you can define a multi-line Haskell function: {{{ Prelude> :set +m Prelude> let Prelude| f :: Int -> Int Prelude| f 0 = 0 Prelude| f n = n - 1 Prelude| Prelude> }}} Is there a reason why it couldn't work with other things like {{{ Prelude> :set -XPatternSynonyms Prelude> let Prelude| pattern A = 'A' Prelude| <interactive>:27:1: error: Illegal pattern synonym declaration for ‘A’ Pattern synonym declarations are only valid at top level }}} {{{ Prelude> let Prelude| data A Prelude| = B Prelude| | C Int Prelude| <interactive>:30:1: error: parse error (possibly incorrect indentation or mismatched brackets) }}} {{{ Prelude> let Prelude| type A = Int Prelude| <interactive>:48:1: error: parse error (possibly incorrect indentation or mismatched brackets) }}} couldn't work, similar to how they work with `:{` and `:}`: {{{ Prelude> :{ Prelude| data A Prelude| = B Prelude| | C Int Prelude| :} Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12833> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13965: COMPLETE sets nerf redundant pattern-match warnings
by GHC 10 Sep '18

10 Sep '18

#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Keywords: | Operating System: Unknown/Multiple PatternSynonyms, | PatternMatchWarnings | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If I give redundant patterns for a datatype with a `COMPLETE` set, then GHC doesn't warn me at all. Here's an example: {{{#!hs {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ViewPatterns #-} module Bug (Boolean(F, TooGoodToBeTrue), catchAll) where data Boolean = F | T deriving Eq pattern TooGoodToBeTrue :: Boolean pattern TooGoodToBeTrue <- ((== T) -> True) where TooGoodToBeTrue = T {-# COMPLETE F, TooGoodToBeTrue #-} catchAll :: Boolean -> Int catchAll F = 0 catchAll TooGoodToBeTrue = 1 catchAll F = 2 }}} GHC thinks this is fine and dandy: {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs -Wall GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, 1 module loaded. }}} This problem also persists across modules: {{{#!hs module Foo where import Bug catchAll2 :: Boolean -> Int catchAll2 F = 0 catchAll2 TooGoodToBeTrue = 1 catchAll2 F = 2 }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Foo.hs -Wall GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 2] Compiling Bug ( Bug.hs, interpreted ) [2 of 2] Compiling Foo ( Foo.hs, interpreted ) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13965> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14912: UnsafeReenter test fails with threaded1 and threaded2
by GHC 10 Sep '18

10 Sep '18

#14912: UnsafeReenter test fails with threaded1 and threaded2 --------------------------------------+--------------------------------- Reporter: alpmestan | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 8.5 Keywords: | Operating System: Linux Architecture: x86_64 (amd64) | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+--------------------------------- {{{ /tmp/ghctest-n4fi8zlk/test spaces/./ffi/should_fail/UnsafeReenter.run UnsafeReenter [bad exit code] (threaded1) /tmp/ghctest-n4fi8zlk/test spaces/./ffi/should_fail/UnsafeReenter.run UnsafeReenter [bad exit code] (threaded2) }}} It's a quite simple program: {{{#!hs {-# LANGUAGE ForeignFunctionInterface #-} -- | Test that unsafe FFI calls crash the RTS if they attempt to re-enter -- Haskell-land module Main where import Foreign foreign import ccall "wrapper" wrap_f :: IO () -> IO (FunPtr (IO ())) foreign import ccall unsafe hello :: FunPtr (IO ()) -> IO () f :: IO () f = putStrLn "Back in Haskell" main :: IO () main = do putStrLn "In Haskell" wrap_f f >>= hello putStrLn "Finished" }}} This just seem to hang (until timeout) with the `threaded1` and `threaded2` ways, instead of erroring out with: {{{ UnsafeReenter: schedule: re-entered unsafely. Perhaps a 'foreign import unsafe' should be 'safe'? }}} which is the expected behaviour. I'll mark the test broken for those 2 ways in an upcoming patch, but this probably deserves a new ticket, so here it is. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14912> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12578: Update links to SPJ’s papers
by GHC 09 Sep '18

09 Sep '18

#12578: Update links to SPJ’s papers -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: Documentation | Version: 8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- It seems that Simon’s homepage got restructured, and a lot of links in the documentation and the code comments are not valid any more. At least these need to be looked at: {{{ $ git grep 'people/simonpj' compiler/types/FamInstEnv.hs:http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/axioms-extended.pdf compiler/types/Unify.hs:[1]: http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/axioms-extended.pdf docs/core-spec/core-spec.mng:phantom equality includes all types. See \url{http://ghc.haskell.org/trac/ghc/wiki/Roles} and \url{http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/coercible.pdf} docs/users_guide/glasgow_exts.rst:paper <http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/>`__. docs/users_guide/glasgow_exts.rst:cloud <http://research.microsoft.com/en- us/um/people/simonpj/papers/parallel/remote.pdf>`__, docs/users_guide/using-optimisation.rst: multiprocessor <http://research.microsoft.com/en- us/um/people/simonpj/papers/parallel/>`__. docs/users_guide/using-optimisation.rst: (ICFP'96) <http://research.microsoft.com/en- us/um/people/simonpj/papers/float.ps.gz>`__. docs/users_guide/using-optimisation.rst: (ICFP'96) <http://research.microsoft.com/en- us/um/people/simonpj/papers/float.ps.gz>`__. docs/users_guide/using-optimisation.rst: programs <http://research.microsoft.com/en-us/um/people/simonpj/papers/spec- constr/index.htm>`__. docs/users_guide/using-optimisation.rst: thesis <http://research.microsoft.com/en-us/um/people/simonpj/papers/santos- thesis.ps.gz>`__ docs/users_guide/using-optimisation.rst: analyser <http://research.microsoft.com/en-us/um/people/simonpj/papers/simple- strictnes-analyser.ps.gz>`__, libraries/base/GHC/Arr.hs:-- http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/coercible.pdf libraries/base/GHC/Base.hs:-- http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/coercible.pdf testsuite/tests/dependent/should_compile/all.T:# http://research.microsoft.com/en-us/um/people/simonpj/papers/haskell- dynamic/ $ git grep '~simonpj' compiler/coreSyn/CoreSyn.hs:-- GHC uses System FC <http://research.microsoft.com/~simonpj/papers/ext-f/> for this purpose, docs/users_guide/bugs.rst: inliner <http://research.microsoft.com/~simonpj/Papers/inlining/>`__. docs/users_guide/glasgow_exts.rst: Languages <http://research.microsoft.com/~simonpj/Papers/slpj-book-1987/>`__. docs/users_guide/glasgow_exts.rst:"group by" <http://research.microsoft.com/~simonpj/papers/list-comp>`__, docs/users_guide/glasgow_exts.rst:space <http://research.microsoft.com/~simonpj/Papers/type-class-design- space/>`__ docs/users_guide/glasgow_exts.rst:papers <http://research.microsoft.com/~simonpj/papers/constraints/index.htm>`__ docs/users_guide/glasgow_exts.rst:Haskell <http://research.microsoft.com/~simonpj/papers/meta-haskell/>`__" docs/users_guide/glasgow_exts.rst:2 <http://research.microsoft.com/~simonpj/papers/meta-haskell/notes2.ps>`__. libraries/base/Control/Concurrent/MVar.hs:-- <http://research.microsoft.com/~simonpj/papers/concurrent-haskell.ps.gz "Concurrent Haskell"> libraries/base/GHC/Conc/Sync.hs:(<http://research.microsoft.com/~simonpj/Papers /asynch-exns.htm>). }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12578> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14330: Sparks are not started promptly
by GHC 08 Sep '18

08 Sep '18

#14330: Sparks are not started promptly -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: sparks | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This was a question on StackOverflow. With some prompting from Yuras, I've decided to open this as an issue. Here is the original question (which has been satisfactorily answered): https://stackoverflow.com/questions/46586941/why-are-ghc-sparks- fizzling/46603680?noredirect=1#comment80163830_46603680 Here is a more narrowly tailored version of the code I have posted there: {{{#!hs {-# LANGUAGE BangPatterns #-} {-# OPTIONS_GHC -O2 -Wall -threaded -fforce-recomp #-} import Criterion.Main import Control.Parallel.Strategies (runEval,rpar,rseq) import qualified Data.Vector.Primitive as PV main :: IO () main = do let fewNumbers = PV.replicate 10000000 1.00000001 manyNumbers = PV.replicate 100000000 1.00000001 defaultMain [ bgroup "serial" [ bench "few" $ whnf serialProduct fewNumbers , bench "many" $ whnf serialProduct manyNumbers ] , bgroup "parallel" [ bench "few" $ whnf parallelProduct fewNumbers , bench "many" $ whnf parallelProduct manyNumbers ] ] serialProduct :: PV.Vector Double -> Double serialProduct v = let !len = PV.length v go :: Double -> Int -> Double go !d !ix = if ix < len then go (d * PV.unsafeIndex v ix) (ix + 1) else d in go 1.0 0 -- | This only works when the vector length is a multiple of 4. parallelProduct :: PV.Vector Double -> Double parallelProduct v = runEval $ do let chunk = div (PV.length v) 4 p2 <- rpar (serialProduct (PV.slice (chunk * 1) chunk v)) p3 <- rpar (serialProduct (PV.slice (chunk * 2) chunk v)) p4 <- rpar (serialProduct (PV.slice (chunk * 3) chunk v)) p1 <- rseq (serialProduct (PV.slice (chunk * 0) chunk v)) rseq (p1 * p2 * p3 * p4) }}} We can build and run this with: {{{ > ghc -threaded parallel_compute.hs > ./parallel_compute +RTS -N6 }}} On my eight-core laptop, here are the results we get: {{{ benchmarking serial/few time 11.46 ms (11.29 ms .. 11.61 ms) 0.999 R² (0.998 R² .. 1.000 R²) mean 11.52 ms (11.44 ms .. 11.62 ms) std dev 222.8 μs (140.9 μs .. 299.6 μs) benchmarking serial/many time 118.1 ms (116.1 ms .. 120.0 ms) 1.000 R² (1.000 R² .. 1.000 R²) mean 117.2 ms (116.6 ms .. 117.9 ms) std dev 920.3 μs (550.5 μs .. 1.360 ms) variance introduced by outliers: 11% (moderately inflated) benchmarking parallel/few time 10.04 ms (9.968 ms .. 10.14 ms) 0.999 R² (0.999 R² .. 1.000 R²) mean 9.970 ms (9.891 ms .. 10.03 ms) std dev 172.9 μs (114.5 μs .. 282.9 μs) benchmarking parallel/many time 45.32 ms (43.55 ms .. 47.17 ms) 0.996 R² (0.993 R² .. 0.999 R²) mean 45.93 ms (44.71 ms .. 48.10 ms) std dev 3.041 ms (1.611 ms .. 4.654 ms) variance introduced by outliers: 20% (moderately inflated) }}} Interestingly, in the benchmark with the smaller 10,000,000 element vector, we see almost no performance improvement from the sparks. But, in the one with the larger 100,000,000 element vector, we see a considerable speedup. It runs 2.5-3.0x faster. The reason for this is that sparks are not started between scheduling intervals. By default, this happens every 20ms. We can see the fizzling like this: {{{ > ./parallel_compute 'parallel/few' +RTS -N6 -s benchmarking parallel/few ... SPARKS: 1536 (613 converted, 0 overflowed, 0 dud, 42 GC'd, 881 fizzled) ... > ./parallel_compute 'parallel/many' +RTS -N6 -s benchmarking parallel/many ... SPARKS: 411 (411 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) ... }}} For application developers, it's possible to work around this by tweaking the scheduling interval: {{{ > ghc -threaded -rtsopts parallel_compute.hs > ./parallel_compute 'parallel/few' +RTS -N6 -s -C0.001 benchmarking parallel/few time 4.158 ms (4.013 ms .. 4.302 ms) 0.993 R² (0.987 R² .. 0.998 R²) mean 4.094 ms (4.054 ms .. 4.164 ms) std dev 178.5 μs (131.5 μs .. 243.7 μs) variance introduced by outliers: 24% (moderately inflated) ... SPARKS: 3687 (3687 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) }}} Much better. But, there are two problems with this: 1. This may negatively impact the overall performance of an application. 2. It doesn't work at all for library developers. It isn't practical to tell end users of your to use certain runtime flags. I don't know enough about the RTS to suggest a way to improve this. However, intuitively, I would expect that if I spark something and there's an idle capability, the idle capability could immediately be given the spark instead of having it placed in the local queue. This may not be possible or may not be compatible with the minimal use of locks in the implementation of sparks though. Here is a comment I made in the StackOverflow thread: > I suppose that in the normal case, if you're going to be sparking things, you should ensure that the work done by all the sparks plus the main thread takes well over 20ms. Otherwise, nearly everything will fizzle unless scheduling happens to be coming soon. I've always wondered about the threshold for how fine-grained sparks should be, and my understanding is now that this is roughly it. In short, I'd like for it to be possible to realize some of the benefits of parallelism for computations that take under 20ms without resorting to `forkIO` and `MVar`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14330> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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