This is factored out part of:
https://golang.org/cl/5279048/
(Parallel GC)

benchmark                             old ns/op    new ns/op    delta
garbage.BenchmarkParser              3999106750   3975026500   -0.60%
garbage.BenchmarkParser-2            3720553750   3719196500   -0.04%
garbage.BenchmarkParser-4            3502857000   3474980500   -0.80%
garbage.BenchmarkParser-8            3375448000   3341310500   -1.01%
garbage.BenchmarkParserLastPause      329401000    324097000   -1.61%
garbage.BenchmarkParserLastPause-2    208953000    214222000   +2.52%
garbage.BenchmarkParserLastPause-4    110933000    111656000   +0.65%
garbage.BenchmarkParserLastPause-8     71969000     78230000   +8.70%
garbage.BenchmarkParserPause          230808842    197237400  -14.55%
garbage.BenchmarkParserPause-2        123674365    125197595   +1.23%
garbage.BenchmarkParserPause-4         80518525     85710333   +6.45%
garbage.BenchmarkParserPause-8         58310243     56940512   -2.35%
garbage.BenchmarkTree2                 31471700     31289400   -0.58%
garbage.BenchmarkTree2-2               21536800     21086300   -2.09%
garbage.BenchmarkTree2-4               11074700     10880000   -1.76%
garbage.BenchmarkTree2-8                7568600      7351400   -2.87%
garbage.BenchmarkTree2LastPause       314664000    312840000   -0.58%
garbage.BenchmarkTree2LastPause-2     215319000    210815000   -2.09%
garbage.BenchmarkTree2LastPause-4     110698000    108751000   -1.76%
garbage.BenchmarkTree2LastPause-8      75635000     73463000   -2.87%
garbage.BenchmarkTree2Pause           174280857    173147571   -0.65%
garbage.BenchmarkTree2Pause-2         131332714    129665761   -1.27%
garbage.BenchmarkTree2Pause-4          93803095     93422904   -0.41%
garbage.BenchmarkTree2Pause-8          86242333     85146761   -1.27%

R=rsc
CC=golang-dev
https://golang.org/cl/5987045