On 06/12/2011 17:48, wren ng thornton wrote:
So, I have an optimization/internals question. Does the GHC API have any
hooks for being able to revert a CAF to the original expression, thus
discarding the previously computed result?
The reason I'm wanting this is that I have a particular CAF which is an
infinite list. Unfolding that list takes a fair deal of work, so we want
to share it whenever possible; however it doesn't take an overwhelming
amount of work, so if we know we've evaluated more of the list than
necessary (for a long while), it'd be nice to be able to revert the
evaluation in order to save on memory overhead (e.g., by calling relax
:: IO()).
I could hack something together based on unsafePerformIO and top-level
IORefs, and it's clear that this is in fact a safe thing to do, but I'm
worried about the semantic issues inherent in unsafePerformIOed
top-level IORefs (e.g., the fact that their scope isn't particularly
well defined: is it per library instance? per runtime?...).
Unfortunately, for what I'm doing, it isn't really feasible to just
leave the IO type in there nor to pass around the infinite list so we
can use scoping rules to decide when to free it.
(Feel free to offer alternative suggestions to handling this situation
too.)
It would be possible, but it's not quite as straightforward as you might
think. Suppose you have a program like this:
xs = [1..100000]
evens = filter ((==0) . (`mod` 2)) xs
and you fully evaluate "evens". Now, GHC will garbage collect "xs",
because it isn't required any more. However, if you revert "evens" to a
CAF, now we require "xs" again, so we have to either revert that to a
CAF or arrange to retain it in the first place on the grounds that we
might need it again if some other CAF is reverted.
Reverting xs to a CAF is not hard - we could have the GC revert CAFs as
soon as they become unreachable. Arranging to retain it is harder.
GHCi gets around this by reverting *all* CAFs at the same time when you
say :load.
There's one other thing: GHC doesn't support reverting CAFs in
interpreted code at the moment, you have to reload the module.
So you need the following things:
- modify the GC to revert CAFs when they become garbage
- add a primop to revert a single CAF
not too hard, I would think...
Cheers,
Simon
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