#13615: Nondeterminism in ‘pure’ function w/ parallel evaluation & memo combinators -------------------------------------+------------------------------------- Reporter: pacak | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): So perhaps we are entering a closure twice and consequently get a race condition where one thread in-place increments a hashtable entry by `x`, then the other does the same, but uses the now already incremented hashtable, resulting in an overall contribution of `2*x`. This explains why the hashtable sum is always larger than the expected result. While it's not clear precisely what closure we are entering multiple times, the fix is clear: ensure that `unordered-containers` is compiled with `-feager-blackholing`. However, we should also (at very least) document this caveat better. The users-guide current advertises `-feager- blackholing` as a optimization to avoid redundant computation in parallel programs. However, in this case that it's absolutely critical for correctness. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13615#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler