[CM] setting heap size preemptively at startup?

Woody Douglass wdouglass at carnegierobotics.com
Fri Sep 10 06:37:51 PDT 2021

I've been doing some GC work as well, and I have a possible feature
that i'm thinking about implementing that i'd like to run by you guys

What if there was a (*s7* 'gc-hook) that ran before the gc routine got
run. From that hook, it should be possible to
1) prevent the gc from running if it's a critical time
2) schedule the gc to run when the system is less busy

I'm using s7 in a video rendering app, and i'd like to only have the gc
run in-between frames, to keep my frame pacing as close to constant as

Let me know what you guys think


On Thu, 2021-09-09 at 21:52 -0700, Iain Duncan wrote:
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> 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.
> 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?
> Thanks so much!
> iain
> On Thu, Sep 9, 2021 at 9:42 PM Elijah Stone <elronnd at elronnd.net>
> wrote:
> > On Thu, 9 Sep 2021, Iain Duncan wrote:
> > 
> > > Is there a way to pre-emptively make the heap big from c or
> > scheme, 
> > > aside from compiling in a default size?
> > (set! (*s7* 'heap-size) ...)
> > 
> > > So I'm wondering if it would be reasonable to give users an
> > option to 
> > > trade size for speed by allocating a big hunk of memory and
> > cleaning up 
> > > when it doesn't matter.
> > That is very reasonable, but note that you cannot shrink the heap.
> > 
> > > does a heap resize take a long time?
> > It requires copying the entire heap.  Probably not something you
> > would 
> > want to do in real-time.
> > 
> > > Do they take progressively more time (ie does the heap double or 
> > > somesuch thing)
> > Yes, it doubles.  (Obviously this also means you're asymptotically
> > less 
> > likely to need a resize as time goes on...)
> > 
> >   -E
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