Cheap ForeignPtr allocation
simonmar at microsoft.com
Wed Sep 4 12:28:07 EDT 2002
> > [...] using a ForeignPtr here, with free() as its finalizer, adds so
> > much overhead that [...]
> Where is the overhead coming from? Is it the cost of a C call or the
> cost of the standard malloc library?
It's the combined cost of
- creating a weak pointer to register the finalizer
- running the finalizer
I can't remember the exact break-down, but I believe more than half the
cost is in malloc+free.
> If the latter, I imagine that a
> custom allocator would have similar performance to using pinned
Yes, using a custom allocator is likely to get you most of the benefit
(I say most, because you'd still need a finalizer and a free() routine,
compared to GC).
But this is an argument in favour of the new interface, because it
abstracts away from the actual allocator used, so the implementor is
free to provide a custom allocator. That's a win, isn't it?
> (I'm sort of assuming that pinned objects are more expensive
> than normal objects.)
> btw I don't know if it's relevant but there's an important semantic
> difference between allocating on the GHC heap and allocating on the C
> heap. The C version can be manipulated by the malloc.h functions. In
> particular, you can call realloc on it. Do that on the GHC heap
> version and... well, it won't be pretty. I don't know if this is
> relevant because using realloc with ForeignPtr'd memory is likely to
> be a delicate procedure no matter how it got allocated.
Yes, I thought about this. Fortunately a ForeignPtr isn't mutable, even
using GHC extensions, so I can't see a way to safely call realloc() once
you've made a ForeignPtr.
Anyway, the docs for mallocForeignPtr would have to say something like
"the pointer is not guarnateed to have been returned by malloc()".
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