解密golang垃圾回收输出

Question

I was running a sample program program using

rahul@g3ck0:~/programs/Remodel$ GOGCTRACE=1 go run main.go 
gc1(1): 0+0+0 ms 0 -> 0 MB 422 -> 346 (422-76) objects 0 handoff
gc2(1): 0+0+0 ms 0 -> 0 MB 2791 -> 1664 (2867-1203) objects 0 handoff
gc3(1): 0+0+0 ms 1 -> 0 MB 4576 -> 2632 (5779-3147) objects 0 handoff
gc4(1): 0+0+0 ms 1 -> 0 MB 3380 -> 2771 (6527-3756) objects 0 handoff
gc5(1): 0+0+0 ms 1 -> 0 MB 3511 -> 2915 (7267-4352) objects 0 handoff
gc6(1): 0+0+0 ms 1 -> 0 MB 6573 -> 2792 (10925-8133) objects 0 handoff
gc7(1): 0+0+0 ms 1 -> 0 MB 4859 -> 3059 (12992-9933) objects 0 handoff
gc8(1): 0+0+0 ms 1 -> 0 MB 4554 -> 3358 (14487-11129) objects 0 handoff
gc9(1): 0+0+0 ms 1 -> 0 MB 8633 -> 4116 (19762-15646) objects 0 handoff
gc10(1): 0+0+0 ms 1 -> 0 MB 9415 -> 4769 (25061-20292) objects 0 handoff
gc11(1): 0+0+0 ms 1 -> 0 MB 6636 -> 4685 (26928-22243) objects 0 handoff
gc12(1): 0+0+0 ms 1 -> 0 MB 6741 -> 4802 (28984-24182) objects 0 handoff
gc13(1): 0+0+0 ms 1 -> 0 MB 9654 -> 5097 (33836-28739) objects 0 handoff
gc1(1): 0+0+0 ms 0 -> 0 MB 209 -> 171 (209-38) objects 0 handoff

Help me understand the first part i.e.

0 + 0 + 0 => Mark + Sweep + Clean times

Does 422 -> 346 means that there has been memory cleanup from 422MB to 346 MB? If yes, then how come the memory is been reduced when there was nothing to be cleaned up?

Answer 1

The output is generated from this line: http://golang.org/src/pkg/runtime/mgc0.c?#L2147

So the different parts are:

• 0+0+0 ms : mark, sweep and clean duration in ms
• 1 -> 0 MB : heap before and after in MB
• 209 - 171 : objects before and after
• (209-38) objects : number of allocs and frees

handoff (and in Go 1.2 steal and yields) are internals of the algorithm.