golang cpu usage in docker

Golang CPU usage in docker

// test.go
package main

import (
	"fmt"
	_ "go.uber.org/automaxprocs"
	"runtime"
	"time"
)

func main() {
	fmt.Println(runtime.NumCPU())
	time.Sleep(time.Second)

	fmt.Println("test")
}

/wip # GODEBUG=schedtrace=100,scheddetail=1 ./auto_maxprocs
SCHED 0ms: gomaxprocs=8 idleprocs=6 threads=5 spinningthreads=1 idlethreads=1 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=1 syscalltick=0 m=0 runqsize=0 gfreecnt=0
  P1: status=0 schedtick=1 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P2: status=1 schedtick=0 syscalltick=0 m=4 runqsize=0 gfreecnt=0
  P3: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P4: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P5: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P6: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P7: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  M4: p=2 curg=-1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 spinning=true blocked=false lockedg=-1
  M3: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=true lockedg=-1
  M2: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 spinning=false blocked=false lockedg=-1
  M1: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 spinning=false blocked=false lockedg=-1
  M0: p=-1 curg=1 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=1
  G1: status=3(chan receive) m=0 lockedm=0
  G2: status=4(force gc (idle)) m=-1 lockedm=-1
  G3: status=4(GC sweep wait) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
2019/06/25 04:45:16 maxprocs: Updating GOMAXPROCS=1: determined from CPU quota
8
SCHED 107ms: gomaxprocs=1 idleprocs=1 threads=5 spinningthreads=0 idlethreads=3 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=1 syscalltick=29 m=-1 runqsize=0 gfreecnt=0
  M4: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=true lockedg=-1
  M3: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=true lockedg=-1
  M2: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=true lockedg=-1
  M1: p=-1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 spinning=false blocked=false lockedg=-1
  M0: p=-1 curg=5 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=true lockedg=-1
  G1: status=4(sleep) m=-1 lockedm=-1
  G2: status=4(force gc (idle)) m=-1 lockedm=-1
  G3: status=4(GC sweep wait) m=-1 lockedm=-1
  G4: status=4(finalizer wait) m=-1 lockedm=-1
  G5: status=3() m=0 lockedm=-1
// ...

if not limit GOMAXPROCS like raw_maxprocs.go, golang will use runtime.NumCPU() as default
with limit GOMAXPROCS like automaxprocs does, after executing init function, the GOMAXPROCS is set to docker cpu limit and this process requires some time.
but after test large computing work with docker cpu limit 1 and change GOMAXPROCS set, if watch from docker stats, the cpu usage is only 100% but it was running on GOMAXPROCS cores with 100%/GOMAXPROCS cpu usage.

so cpu limit in docker can limit the cpu usage of golang but the whole process will be running on GOMAXPROCS (default = runtime.NumCPU()) cores with a CPU usage = limit / GOMAXPROCS, which will cost much on context switch and leads to a poor performance, then auto-maxprocs is useful to avoid this performance loss.

Harold2017/go_cpu.md

Golang CPU usage in docker