The Glasgow Haskell Users Mailing List

Akio Takano | 24 Oct 2012 09:25

Why are arrays retainers?

Hi,

Recently I was surprised to see that GHC's retainer profiler treated
boxed arrays as retainer objects. Why are they considered retainers?

I was using the +RTS -hr to figure out why there were a lot of objects
of type T in the heap. However, the T objects happened to be in a
HashMap (from the unordered-containers package), so the profiler
pointed to the function that created the HashMap rather than the
actual cause of the leak. This lead me to think that it might be
useful if GHC would treat arrays as non-retainers.

Also the behavior is not documented. The user's guide only mentions
stacks and thunks as types of retainers, not mutable objects and boxed
arrays. I'd be happy to submit a documentation patch.

Regards,
Takano Akio

headers

Simon Marlow | 30 Oct 2012 15:41

Re: Why are arrays retainers?

On 24/10/2012 08:25, Akio Takano wrote:

> Recently I was surprised to see that GHC's retainer profiler treated
> boxed arrays as retainer objects. Why are they considered retainers?

I think the intention is to treat all "mutable" objects as retainers, 
where thunks are a kind of mutable object.  From the code it looks like 
both MutableArray# and Array# are treated as retainers though, which is 
probably a mistake.  I'll fix it so that Array# is not a retainer, and 
update the docs to mention that mutable objects are retainers.

Cheers,
	Simon

headers

Akio Takano | 31 Oct 2012 01:05

Re: Why are arrays retainers?

I see. Thank you very much for your explanation and fixing.

- Akio