While #152 is still open, we need to allow for moderately sized allocations
to succeed. Extend the segment size to 256k, which allows for threads to
be allocated.
Fixes#151.
To free memory, we need to allocate memory. In lsa compaction, we convert
N segments with average occupancy of (N-1)/N into N-1 new segments. However,
to do that, we need to allocate segments, which we may not be able to do
due to the low memory condition which caused us to compact anyway.
Fix by introducing a segment reserve, which we normally try to ensure is
full. During low memory conditions, we temporarily allow allocating from
the emergency reserve.
When LSA reclaimer cannot reclaim more space by compaction, it
will reclaim data by evicting from evictable regions.
Currently the only evictable region is the one owned by the row cache.
Requiring alignment means that there must be 64K of contiguous space
to allocate each 32K segment. When memory is fragmented, we may fail
to allocate such segment, even though there's plenty of free space.
This especially hurts forward progress of compaction, which frees
segments randomly and relies on the fact that freeing a segment will
make it available to the next segment request.
