But not compaction.
When reclaiming segments to seastar non-empty segments are copied
as-is to some other place. Instead of doing this reclaimer can copy
only allocated objects and leave the freed holes behing, i.e. -- do
the regular compaction. This would be the same or better from the
timing perspective, and will help to avoid yet another compaction
pass over the same set of objects in the future.
Current migration code checks for the free segments reserve to be
above minimum to proceed with migration, so does the code after this
patch, thus the segment compaction is called with non-empty free
segments set and thus it's guaranteed not to fail the new segment
allocation (if it will be required at all).
Plus some bikeshedding patches for the run-up.
tests: unit(dev)
* https://github.com/xemul/scylla/tree/br-logalloc-compact-on-reclaim-2:
logalloc: Compact segments on reclaim instead of migration
logallog: Introduce RAII allocation lock
logalloc: Shuffle code around region::impl::compact
logalloc: Do not lock reclaimer twice
logalloc: Do not calculate object size twice
logalloc: Do not convert obj_desc to migrator back and forth
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