py/gc - dynamic alloc threshold

[mzakharocsc]

Problem: gc_alloc_threshold in py/gc.c is set statically. however, this is a probllem under memory pressure, amount free after sweep can be less than gc_allloc_threshold amount - essentially rendering this feature useless at preventing memory fragmentation.

Proposal: dynamic gc_alloc_threshold. After sweeping, in gc_alloc(), adjust gc_alloc_threshold to be some fraction of free (2/3, 3/4) -etc (configurable). In our testing 2/3 works really well.

 MP_STATE_MEM(gc_alloc_threshold) = info.free * 2 / 3 / MICROPY_BYTES_PER_GC_BLOCK;

With the proposed change, our production firmware was able to operate without memory fragmentation issues under much lower memory budget.

[Josverl]

Looks interesting,
Do you have a (draft) PR to share to allow a more concrete insight and discussion on the tradeoffs that are likely involved ?

[mzakharocsc]

Please see below for minimum implementation: Missing how to parametrise threshold fraction parameters (hard coded to 2/3):
master...mzakharocsc:micropython:gc_threshold