clearing GHCi (and, by extension, hint) loaded module dependencies

Simon Marlow marlowsd at
Tue Oct 5 10:50:28 EDT 2010

On 05/10/2010 14:32, Alex Suraci wrote:
> On Oct 5, 2010, at 9:25 AM, Simon Marlow wrote:
>> On 26/09/2010 18:14, Alex Suraci wrote:
>>> On Sep 23, 2010, at 7:11 AM, Simon Marlow wrote:
>>>> If you are making separate runGhc calls and still don't see the
>>>> memory being reclaimed, then there might be a leak - but it's hard
>>>> to see where, since everything is reachable from the Session only.
>>>> I suppose we have some global linker state which might be holding
>>>> onto stuff, but as you say that loads are faster in subsequent
>>>> runs, that implies that you're using the same Session.
>>> Hm, I'm not doing any explicit Session handling/reusing. A new hint
>>> interpreter is run with every "load", and nothing is saved or
>>> continued between "load"s; by the time the interpreted function is
>>> executed, the interpreter has completed and its session is no longer
>>> needed. If the Session is sticking around anywhere, I'm not sure
>>> where it would be.
>>> Slightly verbose experimentation results:
>>> As an experiment I just tried adding hint's InterpreterT onto my
>>> language's VM stack, so that it's always running one continuous
>>> interpreter, and calling "reset" after interpreting the "load"
>>> function. An empty script bumped the usage by ~70MB, and subsequent
>>> loads of the same script had no discernible increase and completed
>>> instantly. (It seems the "reset" hint function has no effect on
>>> this.)
>>> Loading another script after the empty one (one that actually does
>>> something - an interface for the Snap webserver) only bumped up the
>>> RAM usage modestly (30MB), implying that some of its dependencies
>>> were already loaded. Doing the reverse (snap script ->   empty script)
>>> jumped the RAM usage up by ~100MB initially, and loading the empty
>>> script repeatedly had no increase and completed instantly.
>>> Repeating these tests with separate runInterpreter calls (how it was
>>> before) has the same characteristics, except that repeated loads of
>>> already-loaded scripts bump up the RAM usage slightly (~8-10MB for
>>> empty, ~20-40MB for snap).
>> How are you measuring the memory usage, btw?
>> Cheers,
>> 	Simon
> I'm just looking in Activity Monitor and filtering my app's name, repeating the
> tests a few times to be sure. I can try again with some other method if you'd
> like.
> Also: I'll be putting together a smaller test-case soon.

Oh, in that case the results are to be expected.  GHC prior to version 
7.0.1 didn't release memory back to the OS even when the memory 
requirements of the program dropped: see

In 7.0.1 we do release memory back to the OS, although I think on 
Windows we're still not releasing address space.


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