golang開啓GODEBUG gctrace =1 顯示信息的含義

golang開啓GODEBUG gctrace =1 顯示信息的含義

標籤（空格分隔）： golang

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如何開啓打印gc信息

只要在程序執行以前加上環境變量GODEBUG gctrace =1 ，如：
GODEBUG gctrace =1 ./xxxx.exe or GODEBUG gctrace =1 go run main.go

程序將會顯示gc信息，以下

gc 1 @2.104s 0%: 0.018+1.3+0.076 ms clock, 0.054+0.35/1.0/3.0+0.23 ms cpu, 4->4->3 MB, 5 MB goal, 4 P
gc 2 @2.241s 0%: 0.019+2.4+0.077 ms clock, 0.079+0/2.4/6.4+0.30 ms cpu, 5->6->5 MB, 6 MB goal, 4 P
gc 3 @2.510s 0%: 0.011+3.2+0.063 ms clock, 0.047+0.10/2.9/9.0+0.25 ms cpu, 11->11->10 MB, 12 MB goal, 4 P
gc 4 @3.021s 0%: 0.013+6.6+0.076 ms clock, 0.053+0.34/6.2/18+0.30 ms cpu, 21->21->20 MB, 22 MB goal, 4 P
gc 5 @3.725s 0%: 0.015+15+0.079 ms clock, 0.062+0.35/15/45+0.31 ms cpu, 40->40->39 MB, 41 MB goal, 4 P
gc 6 @4.741s 0%: 0.008+35+0.17 ms clock, 0.035+0.19/35/100+0.70 ms cpu, 76->76->75 MB, 78 MB goal, 4 P
gc 7 @6.688s 0%: 0.020+117+0.34 ms clock, 0.082+11/117/330+1.3 ms cpu, 147->148->146 MB, 151 MB goal, 4 P
gc 8 @68.645s 0%: 0.019+146+0.30 ms clock, 0.078+0.006/146/407+1.2 ms cpu, 285->285->248 MB, 292 MB goal, 4 P
scvg0: inuse: 426, idle: 0, sys: 427, released: 0, consumed: 427 (MB)
gc 9 @175.448s 0%: 0.030+60+0.12 ms clock, 0.12+0.013/60/177+0.51 ms cpu, 484->484->248 MB, 496 MB goal, 4 P
gc 10 @236.621s 0%: 0.006+59+0.11 ms clock, 0.025+0/59/173+0.47 ms cpu, 484->484->248 MB, 496 MB goal, 4 P
gc 11 @285.967s 0%: 0.027+57+0.22 ms clock, 0.11+0/57/163+0.89 ms cpu, 484->484->248 MB, 496 MB goal, 4 P
scvg1: inuse: 331, idle: 175, sys: 507, released: 0, consumed: 507 (MB)
gc 12 @333.817s 0%: 0.009+52+0.18 ms clock, 0.036+0/52/155+0.72 ms cpu, 484->484->248 MB, 496 MB goal, 4 P

gc信息的含義

查看官方的runtime說明：

gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard
error at each collection, summarizing the amount of memory collected and the
length of the pause. Setting gctrace=2 emits the same summary but also
repeats each collection. The format of this line is subject to change.
Currently, it is:
	gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # P
where the fields are as follows:
	gc #        the GC number, incremented at each GC
	@#s         time in seconds since program start
	#%          percentage of time spent in GC since program start
	#+...+#     wall-clock/CPU times for the phases of the GC
	#->#-># MB  heap size at GC start, at GC end, and live heap
	# MB goal   goal heap size
	# P         number of processors used
The phases are stop-the-world (STW) sweep termination, concurrent
mark and scan, and STW mark termination. The CPU times
for mark/scan are broken down in to assist time (GC performed in
line with allocation), background GC time, and idle GC time.
If the line ends with "(forced)", this GC was forced by a
runtime.GC() call and all phases are STW.

Setting gctrace to any value > 0 also causes the garbage collector
to emit a summary when memory is released back to the system.
This process of returning memory to the system is called scavenging.
The format of this summary is subject to change.
Currently it is:
	scvg#: # MB released  printed only if non-zero
	scvg#: inuse: # idle: # sys: # released: # consumed: # (MB)
where the fields are as follows:
	scvg#        the scavenge cycle number, incremented at each scavenge
	inuse: #     MB used or partially used spans
	idle: #      MB spans pending scavenging
	sys: #       MB mapped from the system
	released: #  MB released to the system
	consumed: #  MB allocated from the system

垃圾回收信息
gc 1 @2.104s 0%: 0.018+1.3+0.076 ms clock, 0.054+0.35/1.0/3.0+0.23 ms cpu, 4->4->3 MB, 5 MB goal, 4 P。

1 表示第一次執行
@2.104s 表示程序執行的總時間
0% 垃圾回收時間佔用的百分比，（不知道和誰比？難道是和上面的程序執行總時間，這樣比較感受沒意義）
0.018+1.3+0.076 ms clock 垃圾回收的時間，分別爲STW（stop-the-world）清掃的時間, 併發標記和掃描的時間，STW標記的時間
0.054+0.35/1.0/3.0+0.23 ms cpu 垃圾回收佔用cpu時間
4->4->3 MB 堆的大小，gc後堆的大小，存活堆的大小
5 MB goal 總體堆的大小
4 P 使用的處理器數量

系統內存回收信息,這個很直白，看單詞就知道大概意思了
scvg0: inuse: 426, idle: 0, sys: 427, released: 0, consumed: 427 (MB)

426 使用多少M內存
0 剩下要清除的內存
427 系統映射的內存
0 釋放的系統內存
427 申請的系統內存