Documentation

The RTS's idle GC doesn't work for us:

• when `-I` is too low it may fire continuously causing scary high CPU when idle among other issues (see #2565)
• when we set it higher it won't run at all leading to memory being retained when idle (especially noticeable when users are benchmarking and see memory stay high after finishing). In the theoretical worst case there is such low haskell heap pressure that we never run finalizers to free the foreign data from e.g. libpq.
• as of GHC 8.10.2 we have access to `-Iw`, but those two knobs still don’t give us a guarantee that a major GC will always run at some minumum frequency (e.g. for finalizers)

...so we hack together our own using GHC.Stats, which should have insignificant runtime overhead.

NOTE: as always the cost of a major GC (forced here, or initiated by the RTS) with the default copying collector is proportional to live (non-garbage) heap data. Tune parameters here to balance: more frequent GC pauses vs. prompt cleanup of foreign data (which does not exert GC pressure).

NOTE: larger nursery size (+RTS -A) may help us run more finalizers during cheaper minor GCs, before they are promoted, making it feasible (maybe) to run this with longer interval parameters.