[I am reading ghc-bugs via the list archives, Cc'ing me would make me
see responses faster]
Here's a simple memoization function:
applyMemo :: (Eq a1,Show a1) = (a1 - b) - MVar [(a1,b)] - (a1 - b)
applyMemo f refTable x1 = unsafePerformIO $ do
-- print x1
table - takeMVar refTable
let key = x1
case lookup key table of
Nothing - do
let result = f x1
putMVar refTable $! (key,result):table
return result
Just memo - putMVar refTable table return memo
The code above is a cut-down example. Initially, I tried to use
Data.HashTable, then IORef (Data.Map ...), then MVars. However, what
remains is that I get funny results: loop, thread blocked
indefinitely, hangs, depending on the exact implementation of
applyMemo, whether I use ghci or ghc, and which function I memoize
(something with a more interesting call pattern than fib).
This is with ghc-6.6 from haskell.org on MacOSX/Intel, same hangs on
amd64-linux.
I have some speculation what's going wrong, namely that one version
of the unsafePerformIO code is interrupted by another (triggered by
some lazy computation), the latter then blocks on the MVar, and the
RTS cannot proceed.
Oh, and uncommenting the print statement makes the error go away,
presumably because x1 is fully evaluated?
Anyway, this code is more or less lifted from the Stretching the
Storage Manager paper [1], and since then there have been quite some
invasive changes to the RTS, if I am not mistaken.
What am I doing wrong, and how can I do it right? :)
(If pressed I could provide actual code which triggers the bug, but
it will take me a while to extract it, and also I would rather have
it not public for now.)
Cheers,
Michael
[1] http://research.microsoft.com/Users/simonpj/Papers/weak.ps.gz
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