[Python-Dev] Proposal: Run GC less often

[Neil Schemenauer]

Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Martin v. Löwis wrote:
>
>> Under my proposal, 10 middle collections must have passed,
>> PLUS the number of survivor objects from the middle generation
>> must exceed 10% of the number of objects in the oldest
>> generation.
>
> What happens if the program enters a phase where it's not
> producing any new cyclic garbage, but is breaking references
> among the old objects in such a way that cycles of them
> are being left behind? Under this rule, the oldest
> generation would never be scanned, so those cycles would
> never be collected.

Another problem is that the program could be slowly leaking and a
full collection will never happen.

>> As a consequence, garbage collection becomes less frequent
>> as the number of objects on the heap grows
>
> Wouldn't it be simpler just to base the collection frequency
> directly on the total number of objects in the heap?
>  From what another poster said, this seems to be what
> emacs does.

I like simple.  The whole generational collection scheme was dreamed
up by me early in the GC's life.  There was not a lot of thought or
benchmarking put into it since at that time I was more focused on
getting the basic GC working.  At some later point some tuning was
done on the collection frequencies but that 10 middle collections
scheme was never deeply investigated, AFAIK.

BTW, I suspect that documentation needs updating since I understand
that the GC is no longer optional (the stdlib and/or the Python
internals create reference cycles themselves).

  Neil