Your message dated Tue, 23 Jan 2007 15:05:16 +0000
with message-id <[EMAIL PROTECTED]>
and subject line randomRs now runs in constant space
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Package: ghc6
Version: 6.4.1-1
Severity: normal
I was writing a program that needed large quantities of random numbers,
and so I used randomRs to generate a stream that I could consume:
map (lineChars!) $ randomRs (bounds lineChars) g
lineChars is of course an array that I'm pulling elements from. Imagine
my surprise when, within seconds of starting the compiled program, it had
eating 80% of my virtual memory space! After learning how to use the GHC
heap profiler, I was able to trace my troubles to the above expression.
On a whim, I replaced it with the following expression (which ought to be
equivalent):
map (lineChars!) $ unfoldr (Just . randomR (bounds lineChars)) g
As soon as I did so, the memory leak vanished. I don't understand why
randomRs is eating tons of heap here, but it seems pretty clear that it's
the culprit.
I've attached the full program for inspection; it generates a number of
lines (the number is given on the command-line) of random characters and
spits them to stdout. The leak becomes pronounced when you get to around
50,000 to 100,000 lines of output. The "toSplit" binding is the one that
leaks; I've included several alternative definitions, none of which leak.
Daniel
-- System Information:
Debian Release: testing/unstable
APT prefers unstable
APT policy: (500, 'unstable'), (1, 'experimental'), (1, 'unstable')
Architecture: i386 (i686)
Shell: /bin/sh linked to /bin/bash
Kernel: Linux 2.6.15-1-686
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Versions of packages ghc6 depends on:
ii gcc 4:4.0.2-2 The GNU C compiler
ii haskell-utils 1.6.0.1 Utilities used by the Debian Haske
ii libc6 2.3.5-12 GNU C Library: Shared libraries an
ii libgmp3-dev 4.1.4-11 Multiprecision arithmetic library
ii libgmp3c2 4.1.4-11 Multiprecision arithmetic library
ii libncurses5 5.5-1 Shared libraries for terminal hand
ii libreadline5 5.1-5 GNU readline and history libraries
ii libreadline5-dev 5.1-5 GNU readline and history libraries
ii perl [perl5] 5.8.7-10 Larry Wall's Practical Extraction
ghc6 recommends no packages.
-- no debconf information
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--- Begin Message ---
Version: 6.6-1
As noted earlier, randomRs runs in constant space in 6.6.
Thanks
Ian
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