[Haskell-cafe] Space leaks

Peter Gavin pgavin at gmail.com
Thu Jul 17 23:00:26 EDT 2008

Thanks for the responses.

This is basically what I've got looks like (grossly simplified):

data Monad m => Foo m a b =
      { action :: m (Foo m a b, b)
      , update :: a -> Foo m a b

The driver loop injects new values with update, and executes action 
whenever it's ready to, replacing the old Foo with the newly returned Foo.

I finally fixed the space leak it by inserting SPECIALIZE pragmas for 
Foo IO a b on every function that creates a Foo. I'm not sure if I can 
remove all the strictness annotations I've accumulated yet, though. 
This is a bit disconcerting, though, because in the future I'd like to 
not use IO and use a strict State instead. I hope I won't have to 
specialize for every monad that ends up getting used.

Thanks again,

Jefferson Heard wrote:
> Peter, from 500 feet, we can't see much, but your strictness might
> actually be your problem depending on what "largish" looks like and
> whether you're reading your data from disc.  It's entirely possible
> that your data structure updates or disc reads are head-strict and
> you're evaluating or loading the entirety of data in memory at a
> single update.
> -- Jeff
> On Thu, Jul 17, 2008 at 2:58 PM, Luke Palmer <lrpalmer at gmail.com> wrote:
>> On Thu, Jul 17, 2008 at 12:14 PM, Peter Gavin <pgavin at gmail.com> wrote:
>>> Hello everyone,
>>> I have this piece of code I've been working on, and I've been stuck on
>>> tracking down a space leak in it for some time now.  The code is essentially
>>> a tight loop that updates a rather largish data structure with embedded
>>> functions that are called by the driver loop.  The code doesn't accumulate
>>> any data as the loop runs (at least deliberately), so I would expect the
>>> memory profile to be flat.  Unfortunately, the profile is a wedge :)   I've
>>> added bangs and `seq` literally everywhere, and it looks (to me at least)
>>> like there's nothing left to be lazily evaluated anywhere.  I've used
>>> retainer profiling, and the functions that are leaking space according to
>>> the profiler output are strict throughout.
>> I don't know what I can suggest as for general tactics.  Without
>> seeing the code it's hard to say what could be happening.  Just
>> remember that strictness is not always the answer!
>> >From the very limited amount of information I got from this
>> description, my first guess would be the data structure itself, or the
>> functions inside it.  If it's lazily generated, then you might not be
>> seeing the full amount of space it's taking up at once.  But that's
>> just a guess.
>> Luke
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