MPS: dangling markers

[Ihor Radchenko <yantar92@posteo.net>]

Related: bug#71644, bug#63040

When running the scratch/igc branch, I noticed a severe slowdown (10x or
more) of redisplay on my large Org buffers.

- The redisplay manifests itself as I build agenda multiple times
  (agenda creates many new markers).

- perf points to (1) itree lookup; (2) bytepos<->charpos, which implies
  that things may be related to the number of allocated markers and
  overlays.

- I observe things slowing down as the number of "PVEC_MARKER" in the
  output of `igc-info' grows.

- Forcing (igc--collect) returns speed to normal

I believe that my observation reveals one important point that may need
to be considered when using MPS: 

We may need to more careful about cases in Emacs C code that traverse
object lists that may contain unreferenced objects.  Because MPS does
not perform GC as regularly as the traditional mark-and-sweep, some
unreferenced objects may remain live and present in internal data
structures for a long time.

AFAIK, at least overlays, buffers, and markers are used in C code within
object lists/trees are often traversed in full. If objects in these
lists are not regularly garbage-collected, we may end up in situation
when dead objects significantly impact performance. I believe that what
I observe locally is exactly the described scenario.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
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