I think this is at least partially due to the way parallel compaction deals with
the young gen. When copying from a young gen space (e.g., eden or from) to the
old gen, *all* live objects in the space must fit into the old gen; otherwise,
none of them will be copied. This is because par compaction operates on
'regions' of the heap and copying only part of a space would require extra
bookkeeping (or some good luck).
A scavenge, if it were done, would be able to promote some things into the
old gen. Even though scavenge before full gc is enabled, the scavenge code
bails out because the estimated amount of bytes promoted is > the amount of
free space in the old gen (in PSScavenge::should_attempt_scavenge()).
I expected the gc overhead limit to kick in, since we are spending nearly all
our time in GC and not collecting anything. We should get an OOME because we
are spending too much time in GC. Update after a conversation with the
implemtor of the overhead limit: the overhead limit is not triggered because
there is too much free space in the old generation (more than 2%).
Could modify the policy so that when the estimated bytes to promote is > the
free space in the old gen, a scavenge is skipped only if the last scavenge
was not skipped for the same reason. In other (simpler) words, don't skip