More granular chunking
More granular chunking

Summary:
Check some statistics of allocations for the main thread (the only one consistently running). It turns out that the power-of-two sizing has significant padding overhead that isn't made up for by holding less 8x blocks.

Switch to having 4-aligned blocks up to 1024, then 1024-aligned blocks up to 64K. Finally, for sizes of 1M or more, only store four blocks.

Reviewed By: Kewen12

Differential Revision: D32481744

fbshipit-source-id: 68cafa290eb309695a9db62f10ccd68791ebcd44

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