runtime: add per-M caches for MemStats Avoid touching centralized state during memory manager operations.
runtime: add per-M caches for MemStats
Avoid touching centralized state during
memory manager operations.

R=mirtchovski
CC=golang-dev, rsc
http://codereview.appspot.com/4766042

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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