On 01/02/12 10:44, Phil Clayton wrote:
On 29/01/12 15:57, David Matthews wrote:
On 27/01/2012 20:15, Phil Clayton wrote:
Thinking about explicitly deleting callbacks led me to ask whether each
ML function passed as a callback has its callback reused on subsequent
calls. If reuse occurs, the number of call sites in the source code
would bound memory usage.
I'm guessing that these callback objects are actually wrappers to invoke
compiled ML code and that there is only one instance of the compiled ML,
so there would be no problem having only one instance of the callback
object.
It would be nice if they could be reused but currently they aren't. I
had a look at the code and did some experiments but the problem is that
there is quite a lot of ML wrapping going on. That means that the RTS
can't detect that the same function has been passed a second time. After
the wrapping has been applied the closure that gets passed into the RTS
is always different even if the same ML function was used further up.
I sometimes think the whole FFI system should be redesigned to remove
some of the layers.
Thanks for investigating. It looks like every callback is leaking
something like 0x38 bytes of memory which isn't an issue in the short
term while I am only developing prototype GTK applications. (An
application would need to run for days, if not weeks, for this to cause
problems.)
This may be more of an issue than I first thought. I am looking at some
profiling output which is indicating that finalizers are not being
called for certain vols. These vols are precisely those that are
referenced by functions passed to C as callbacks. Presumably, because
callbacks are never garbage collected, the callback functions are always
reachable, as is everything that they reference. This would explain why
the finalizers are never called for those vols and, presumably, means
that no ML functions used as callbacks are garbage collected. If I
understand correctly, this means a lot more memory is effectively being
leaked than I previously thought.
I think there definitely needs to be a way to delete a callback
explicitly so garbage collection can clean up and call finalizers.
Poly/ML just can't know how long a callback needs to be kept - the
application must tell it. However, can the deletion occur from the C
side given the callback function pointer? (Naturally the C side should
have no pointer into the ML heap.)
Phil
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