runtime.gogc configures GOGC (the GC target percentage).

Setting runtime.gogc is equivalent to setting the GOGC environment variable which also takes precedence over the value from the config file.

Increasing this value means fewer but longer GC cycles since there is more garbage to collect.

The default of GOGC=100 works for Go 1.4 but shows a significant performance drop for Go 1.5 since the concurrent GC kicks in more often.

During benchmarking I have found the following values to work for my setup and for now I consider them sane defaults for both Go 1.4 and Go 1.5.

GOGC=100: Go 1.5 40% slower than Go 1.4
GOGC=200: Go 1.5 == Go 1.4 with GOGC=100 (default)
GOGC=800: both Go 1.4 and 1.5 significantly faster (40%/go1.4, 100%/go1.5)

The default is

runtime.gogc = 800