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Right now, G1 uses fixed size PLABs for allocation during GC (set by the imaginatively-named parameter -XX:ParallelGCG1AllocBufferSize). It'd be nice to do what CMS does and use dynamic ergonomic resizing of the PLABs.
This might have dependency on 6722568, as if PLABs are very large and appear as a single allocation to the BOT might impose a non-trivial scanning overhead which might cancel out any benefits we'll get out of using large PLABs in the first place.
Performance anomalies being currently investigated.