[CM] [SPAM:##] Re: setting heap size preemptively at startup?

[Elijah Stone]

On Thu, 9 Sep 2021, Iain Duncan wrote:

> Thanks Elijah, that's very helpful. So this is what I'm taking away from what you said, if you don't mind correcting me anywhere I'm wrong, that would be lovely
> - we can lock out the gc for the duration of a piece, this is reasonable
> - but to do so, we should definitely establish the upper bound of the heap size you need and pre-allocate it
> - we really don't want to get this wrong, because doubling the heap part way through will be (probably) an underrun, as everything in the old heap gets copied to the new one
> -  once the heap is big, only a reboot of the interpreter will bring it back down again.

Yes


> One thing I'm not clear on: is it necessarily a lot slower for the gc to do a run with a large heap, even if the heap is not in use? Or is the bottleneck the number of objects the gc goes trawling through?
> I guess another way of putting that is: is there any real disadvantage to over-allocating the heap size aside from using up RAM?

This is where my GC knowledge is showing its limits!

I think that it is proportional to the amount of heap space that you 
actually use.  But this depends not only on how much you cons, but also on 
how fragmented the heap is (since s7 gc is non-compacting), which is 
application-dependent.

  -E