[Haskell-cafe] GC

wren ng thornton wren at freegeek.org
Thu May 7 19:33:11 EDT 2009

Daniel Fischer wrote:
> Am Donnerstag 07 Mai 2009 22:01:11 schrieb Andrew Coppin:
> > Simon Marlow wrote:
> > > http://hackage.haskell.org/trac/ghc/ticket/698
> > I presume that the reason for this is to avoid handing memory back only
> > to immediately need it again? (I.e., we don't want to be constantly
> > asking the OS to allocate and deallocate memory. Allocate it once and
> > then let the RTS handle it.)
> >
> > How hard would it be to add a function to ask the RTS to shrink the
> > allocated memory? E.g., you do something that you know consumes lots of
> > RAM, you finish doing it, you know that your live set has probably gone
> > way down now, so you ask the RTS to release some RAM if possible. Would
> > that be difficult?
> Another idea, I have no idea how hard or sensible it is:
> What if the GC detects how much memory is currently used (it does already do that, doesn't 
> it?) and how much is allocated from the OS, and if less than a third or a quarter of the 
> allocated memory is used, return the memory that exceeds twice the used memory to the OS?
> Behaviour might be configurable by an RTS flag.

+1 (assuming the GC does detect current usage).

I think this behavior (with flags to tune the multiples, or to disable 
it) is nicely high-level and easy for users to reason about.

For users with intimate knowledge of the GC and their program's behavior 
it could be helpful to have the alternative proposal of an impure 
function to give hints to the RTS, but in general this seems like far 
too low-level of an approach to solve the general problem.

Live well,

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