The new implementation features wait-free
fast path for readers which significantly improves
performance/scalability on read-mostly workloads.
Benchmark results on HP Z600 (2 x Xeon E5620, 8 HT cores, 2.40GHz)
are as follows:
benchmark                              old ns/op    new ns/op    delta
BenchmarkRWMutexUncontended               179.00        96.60  -46.03%
BenchmarkRWMutexUncontended-2              89.10        49.10  -44.89%
BenchmarkRWMutexUncontended-4              44.70        24.70  -44.74%
BenchmarkRWMutexUncontended-8              23.30        12.90  -44.64%
BenchmarkRWMutexUncontended-16             16.80         8.75  -47.92%
BenchmarkRWMutexWrite100                   79.60        26.80  -66.33%
BenchmarkRWMutexWrite100-2                305.00        33.00  -89.18%
BenchmarkRWMutexWrite100-4                245.00       113.00  -53.88%
BenchmarkRWMutexWrite100-8                330.00       147.00  -55.45%
BenchmarkRWMutexWrite100-16               371.00       152.00  -59.03%
BenchmarkRWMutexWrite10                    78.30        29.80  -61.94%
BenchmarkRWMutexWrite10-2                 348.00       165.00  -52.59%
BenchmarkRWMutexWrite10-4                 447.00       199.00  -55.48%
BenchmarkRWMutexWrite10-8                 564.00       180.00  -68.09%
BenchmarkRWMutexWrite10-16                492.00       192.00  -60.98%
BenchmarkRWMutexWorkWrite100             1077.00      1037.00   -3.71%
BenchmarkRWMutexWorkWrite100-2            659.00       596.00   -9.56%
BenchmarkRWMutexWorkWrite100-4            509.00       361.00  -29.08%
BenchmarkRWMutexWorkWrite100-8            603.00       351.00  -41.79%
BenchmarkRWMutexWorkWrite100-16           750.00       607.00  -19.07%
BenchmarkRWMutexWorkWrite10               990.00       951.00   -3.94%
BenchmarkRWMutexWorkWrite10-2            1119.00      1070.00   -4.38%
BenchmarkRWMutexWorkWrite10-4            1300.00      1199.00   -7.77%
BenchmarkRWMutexWorkWrite10-8            1424.00      1291.00   -9.34%
BenchmarkRWMutexWorkWrite10-16           1981.00      1786.00   -9.84%

R=rsc
CC=golang-dev
https://golang.org/cl/4671051