runtime: use lock-free ring for work queues
Use lock-free fixed-size ring for work queues
instead of an unbounded mutex-protected array.
The ring has single producer and multiple consumers.
If the ring overflows, work is put onto global queue.
benchmark old ns/op new ns/op delta
BenchmarkMatmult 7 5 -18.12%
BenchmarkMatmult-4 2 2 -18.98%
BenchmarkMatmult-16 1 0 -12.84%
BenchmarkCreateGoroutines 105 88 -16.10%
BenchmarkCreateGoroutines-4 376 219 -41.76%
BenchmarkCreateGoroutines-16 241 174 -27.80%
BenchmarkCreateGoroutinesParallel 103 87 -14.66%
BenchmarkCreateGoroutinesParallel-4 169 143 -15.38%
BenchmarkCreateGoroutinesParallel-16 158 151 -4.43%
R=golang-codereviews, rsc
CC=ddetlefs, devon.odell, golang-codereviews
https://golang.org/cl/
46170044