1. If we fail a humongous allocation, immediately upgrade to full gc.
   There is little likelihood that a degen will remedy this situation.
2. Reduce the likelihood that we will fail a humongous allocation by:
   a) Servicing regular mutator allocation from high heap,  preserving
      low heap for humongous allocations
   b) At end of concurrent old mark, if old-gen memory has become
      scattered throughout low memory, add a percentage of these
      scattered old-gen regions to the set of mixed-evac candidates so
      that old-gen can be defragmented, even if these regions do not
      have a large amount of garbage contained within them.

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