Question

我试图理解在运行从Go（v1.7）程序编译的一组可执行文件的系统上启用GODEBUG=gctrace=1时提到的确切时序字段。

我用来获取统计数据的命令是：

GODEBUG=gctrace=1 ~/golang/go/bin/godoc -http=:5555 -index

现在，文档说：

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.

输出的示例行是：

gc 408 @4005.361s 0%: 0.061+86+1.2 ms clock, 1.9+0/1412/3894+38 ms cpu, 1080->1095->556 MB, 1108 MB goal, 72 P

我想要了解的是与时间相关的字段：

0.061 + 86 + 1.2 ms时钟
1.9 + 0/1412/3894 + 38 ms cpu

根据文件：

STW sweep termination: 0.061 ms
Concurrent mark and scan: 86 ms
STW mark termination: 1.2 ms

现在，第二组时间应该是之前标记/扫描时间的细分。但它们看起来相当大（3894 ms？），我不理解混合中的+。它们表示什么以及故障如何显示大于值？

Answer 1

source code is in runtime/mgc.go：

for i, ns := range []int64{
    sweepTermCpu, 
    gcController.assistTime, 
    gcController.dedicatedMarkTime + gcController.fractionalMarkTime,
    gcController.idleMarkTime, 
    markTermCpu} {
    if i == 2 || i == 3 {
        // Separate mark time components with /.
        print("/")
    } else if i != 0 {
        print("+")
    }
    print(string(fmtNSAsMS(sbuf[:], uint64(ns))))
}

所以1.9+0/1412/3894+38 ms cpu的意思是：

   1.9 sweepTermCpu, 
   0   gcController.assistTime, 
1412   gcController.dedicatedMarkTime + gcController.fractionalMarkTime,
3894   gcController.idleMarkTime, 
  38   markTermCpu

gcController.idleMarkTime是此周期在idle marking中花费的纳秒。
不足为奇的是，GC利用空闲处理器进行某些标记。

// gcMarkWorkerIdleMode indicates that a P is running the mark
// worker because it has nothing else to do. The idle worker
// should run until it is preempted and account its time
// against gcController.idleMarkTime.
gcMarkWorkerIdleMode

实际上有pending issue 14179指出GC做不到足够的（！），应该唤醒空闲的处理器。

gctrace

1 个答案: