Memory accounting is enabled on demand; if demand goes
away --- as approximated by no live custodians having
a limit or previously been queried for memory use ---
then stop accounting until demand resumes.
Fix the case that an old-generation finalizer ends up
on a modified page after all old-generation marking
is complete.
Also, make sure epehemerons are checked after previous
marking that may have left the stack empty.
The allocation strategy for immobile objects avoids some fragmentation
in non-incremental mode, but it interferes with finishing up an
incremental major collection, so trade some fragmentation for
an earlier finish (which is far more likely to use less memory
instead of more, despite extra fragmentation).
Really, just improve when majors GCs are forced to trigger
further finalizations. This improvement makes `(collect-garbage)`
followed by `(collect-garbage 'incremental)` move more
reliably into incremental mode.
When `(collect-garbage 'minor)` is combined with incremental
mode, do less incremental work. At the same time, don't
skip an incremental GC just because a major GC is ready.
Although calling `(collect-garbage 'incremental)` in a program with
a periodic task is the best way to request incremental collection, it's
handy for some experiments to have an environment variable that turns
it on permanently.
This change also makes incremental-mode minor collections log as "mIn"
instead of "min", and it changes the first field of the logged
`gc-info` structure to be a mode symbol instead of a boolean.
Incremental GC now works well enough to be useful for some programs
(e.g., games). Memory accounting is still not incremental, so DrRacket
(and running programs in DrRacket) does not really support incremental
collection, although pause times can be much shorter in incremental
mode than by default.
This step is semi-incremental, in that it can happen during
incremental collection, but all finalization is performed at once.
This will work well enoguh if the number of finalizers, weak boxes,
etc., will be small enough relative to the heap size.
The bug was an induced failure in the http server, to test retry
support, triggered another run of half of the synchronization
protocol, leading to a stuck state.
