《Go 语言运行时原理》2.2 调度循环与 work stealing 实测

GODEBUG=schedtrace=800 ./sched

SCHED 0ms: gomaxprocs=10 idleprocs=8 threads=3 spinningthreads=1 needspinning=0 idlethreads=0 runqueue=0 [ 0 0 0 0 0 0 0 0 0 0 ] schedticks=[ 0 0 0 0 0 0 0 0 0 0 ]
SCHED 811ms: gomaxprocs=10 idleprocs=0 threads=11 spinningthreads=0 needspinning=1 idlethreads=0 runqueue=8 [ 1 0 1 0 1 1 1 0 0 1 ] schedticks=[ 34 30 31 29 31 31 31 31 33 34 ]
SCHED 1624ms: gomaxprocs=10 idleprocs=0 threads=11 spinningthreads=0 needspinning=1 idlethreads=0 runqueue=8 [ 1 1 0 1 0 0 1 1 1 0 ] schedticks=[ 65 62 62 61 61 63 63 63 65 65 ]
SCHED 2427ms: gomaxprocs=10 idleprocs=0 threads=11 spinningthreads=0 needspinning=1 idlethreads=0 runqueue=8 [ 1 1 0 0 0 1 0 1 1 1 ] schedticks=[ 86 83 84 82 82 84 84 84 86 86 ]

samples := []metrics.Sample{
	{Name: "/sched/goroutines:goroutines"},
	{Name: "/sched/goroutines/runnable:goroutines"},
	{Name: "/sched/goroutines/running:goroutines"},
	{Name: "/sched/goroutines/waiting:goroutines"},
	{Name: "/sched/gomaxprocs:threads"},
}
// ... 启动 24 个 CPU 密集型 goroutine，800ms 后读取 ...
metrics.Read(samples)

/sched/goroutines:goroutines                  30
/sched/goroutines/runnable:goroutines         15
/sched/goroutines/running:goroutines          10
/sched/goroutines/waiting:goroutines          5
/sched/gomaxprocs:threads                     10

go test -bench=BenchmarkCPU -benchtime=300x -cpu=1,2,4,8,10 -count=3 .

BenchmarkCPU        	     300	     26460 ns/op
BenchmarkCPU-2      	     300	     13340 ns/op
BenchmarkCPU-4      	     300	      6708 ns/op
BenchmarkCPU-8      	     300	      4073 ns/op
BenchmarkCPU-10     	     300	      4485 ns/op

go test -bench=BenchmarkPingPong -benchtime=200000x -cpu=1,2,4 -count=3 .

BenchmarkPingPong     	  200000	       248.5 ns/op
BenchmarkPingPong     	  200000	       232.6 ns/op
BenchmarkPingPong     	  200000	       215.5 ns/op
BenchmarkPingPong-2   	  200000	       253.9 ns/op
BenchmarkPingPong-2   	  200000	       264.0 ns/op
BenchmarkPingPong-2   	  200000	       246.5 ns/op
BenchmarkPingPong-4   	  200000	       266.0 ns/op
BenchmarkPingPong-4   	  200000	       273.9 ns/op
BenchmarkPingPong-4   	  200000	       283.1 ns/op

// src/runtime/proc.go:4150（片段）
func schedule() {
	mp := getg().m
top:
	pp := mp.p.ptr()
	pp.preempt = false
	...
	gp, inheritTime, tryWakeP := findRunnable() // blocks until work is available
	...
	execute(gp, inheritTime) // Never returns.
}

grep -n "Check the global runnable queue\|runqget(pp)\|Spinning Ms: steal work\|We have nothing to do" proc.go

3455:	// Check the global runnable queue once in a while to ensure fairness.
3484:	if gp, inheritTime := runqget(pp); gp != nil {
3527:	// Spinning Ms: steal work from other Ps.
3555:	// We have nothing to do.

// src/runtime/proc.go:3843（片段）
func stealWork(now int64) (gp *g, inheritTime bool, rnow, pollUntil int64, newWork bool) {
	pp := getg().m.p.ptr()
	ranTimer := false

	const stealTries = 4
	for i := 0; i < stealTries; i++ {
		stealTimersOrRunNextG := i == stealTries-1
		for enum := stealOrder.start(cheaprand()); !enum.done(); enum.next() {
			if sched.gcwaiting.Load() {
				return nil, false, now, pollUntil, true
			}
			p2 := allp[enum.position()]
			if pp == p2 {
				continue
			}
			...
			if !idlepMask.read(enum.position()) {
				if gp := runqsteal(pp, p2, stealTimersOrRunNextG); gp != nil {
					return gp, false, now, pollUntil, ranTimer
				}
			}
		}
	}
	...
}

// src/runtime/proc.go:7781
func runqsteal(pp, p2 *p, stealRunNextG bool) *g {
	t := pp.runqtail
	n := runqgrab(p2, &pp.runq, t, stealRunNextG)
	if n == 0 {
		return nil
	}
	n--
	gp := pp.runq[(t+n)%uint32(len(pp.runq))].ptr()
	...
	return gp
}

// src/runtime/proc.go:7649（片段）
func runqget(pp *p) (gp *g, inheritTime bool) {
	next := pp.runnext
	if next != 0 && pp.runnext.cas(next, 0) {
		return next.ptr(), true // inheritTime=true：继承当前 G 剩余时间片
	}
	for {
		h := atomic.LoadAcq(&pp.runqhead)
		t := pp.runqtail
		if t == h {
			return nil, false
		}
		gp := pp.runq[h%uint32(len(pp.runq))].ptr()
		if atomic.CasRel(&pp.runqhead, h, h+1) {
			return gp, false
		}
	}
}