Golang 抢占调度流程分析

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Golang 抢占调度流程分析

一.前置知识

本文针对调度抢占逻辑的源码阅读,如果配合下列菜单食用效果更佳.

熟悉 golang 的基本知识和语法.
了解目前golang G/M/P 的概念和goroutine
了解用户态线程/stackfull协程的基本原理.
推荐资料:Go Preemptive Scheduler Design Doc
本文环境: go1.12.5 GOOS=darwin GOARCH=amd64

二. 源码分析总

2.1 总体框架

总体思想:

sysmon中定期扫描正在执行的g列表,筛选出执行时间过长的g并且设置需要被抢占的标签.
在恰当的地方检测被抢占标记,(runtime主动)切换,让出cpu.

2.2 标记待抢占G

2.2.1 关键数据结构

sysmontick结构:

顾名思义sysmontick 用来保存 sysmon抢占检测的上下文.

type sysmontick struct {
    schedtick   uint32 //标记某个g的调度次数
    schedwhen   int64 //某个g最近一次的调度时间戳
    syscalltick uint32 //某个g的系统调用次数
    syscallwhen int64 //某个g最近一次系统调用时间戳
}

p结构:

这里只摘取P结构中与抢占相关的字段

type p struct {
    lock mutex
   ......

    status      uint32 // one of pidle/prunning/...
    schedtick   uint32     // incremented on every scheduler call
    syscalltick uint32     // incremented on every system call
    sysmontick  sysmontick //即上面sysmontick,sysmon会操作这个数据,用于表示最近一次的检测上下文.
  ......
}

2.2.2 扫描&标记

sysmon其实干了很多事情,现在只关注两个点:

扫描的周期
标记的逻辑

func sysmon() {
    ......
    idle := 0 // how many cycles in succession we had not wokeup somebody
    delay := uint32(0)
    for {
        if idle == 0 { // start with 20us sleep...
            delay = 20
        } else if idle > 50 { // start doubling the sleep after 1ms...
            delay *= 2
        }
        if delay > 10*1000 { // up to 10ms
            delay = 10 * 1000
        }
        usleep(delay)
    ......
        // retake P's blocked in syscalls
        // and preempt long running G's
        if retake(now) != 0 {
            idle = 0
        } else {
            idle++
        }
    }
}

关于扫描周期,至少是20us一个循环,后面视idle循环次数来进行指数退避(超过1ms之后倍增),但最长时间不超过10ms,故系统至多在10ms左右进行一次抢占检测.

接下来看抢占的具体检测标记逻辑retake

// forcePreemptNS is the time slice given to a G before it is
// preempted.
const forcePreemptNS = 10 * 1000 * 1000 // 10ms

func retake(now int64) uint32 {
    n := 0
    lock(&allpLock)
    for i := 0; i < len(allp); i++ {
        _p_ := allp[i]
        if _p_ == nil {
            continue
        }
        pd := &_p_.sysmontick 
        s := _p_.status 
        if s == _Psyscall {
             ......
        } else if s == _Prunning {
            // Preempt G if it's running for too long.
            t := int64(_p_.schedtick)
      if int64(pd.schedtick) != t {
                pd.schedtick = uint32(t)
                pd.schedwhen = now
                continue
            }
            if pd.schedwhen+forcePreemptNS > now { 
                continue
            }
            preemptone(_p_)
        }
    }
    unlock(&allpLock)
    return uint32(n)
}

通过遍历allp列表来获取正在运行的g.

状态检测.

        t := int64(_p_.schedtick)
      if int64(pd.schedtick) != t { //在周期内已经调度过,即当前p上运行的g改变过.
                pd.schedtick = uint32(t)
                pd.schedwhen = now //更新最近一次抢占检测的时间
                continue
            }
            if pd.schedwhen+forcePreemptNS > now { 
                continue
            }
            preemptone(_p_)

从上面关键数据结构得知 p.schedtick 记录了这个P上总共调度次数(递增), 故sysmon通过比较最近一次记录的schedtick 即可判断在一个周期内是否发生过调度行为.

通过最近一次检测时间与当前时间比较来明确是否需要抢占标记pd.schedwhen+forcePreemptNS>now

forcePreemptNS为10ms ,如果超过10ms没有调度,则需要抢占, PS:并不能保证一个G最多运行10ms.

最后通过preemptone来标记当前G需要被抢占

func preemptone(_p_ *p) bool {
    mp := _p_.m.ptr()
    if mp == nil || mp == getg().m {
        return false
    }
    gp := mp.curg
    if gp == nil || gp == mp.g0 {
        return false
    }
    gp.preempt = true
    // Every call in a go routine checks for stack overflow by
    // comparing the current stack pointer to gp->stackguard0.
    // Setting gp->stackguard0 to StackPreempt folds
    // preemption into the normal stack overflow check.
    gp.stackguard0 = stackPreempt
    return true
}

核心关键就是设置g.preempt=true和g.stackguard0 = stackPreempt.

2.3 抢占触发

func preemptone 注释已经说了(runtime的注释很好),抢占触发时机: 目标g进行函数调用中触发栈检测过程中进行.

func testfunc()(sum int){
    var nums[100] int
    for _, num := range nums {
        sum += num
    }
    return
}

"".testfunc STEXT size=132 args=0x8 locals=0x328
    0x0000 00000 (main.go:60)    TEXT    "".testfunc(SB), ABIInternal, $808-8
    0x0000 00000 (main.go:60)    MOVQ    (TLS), CX
    0x0009 00009 (main.go:60)    LEAQ    -680(SP), AX
    0x0011 00017 (main.go:60)    CMPQ    AX, 16(CX)
    0x0015 00021 (main.go:60)    JLS    122
    0x0017 00023 (main.go:60)    SUBQ    $808, SP
    0x001e 00030 (main.go:60)    MOVQ    BP, 800(SP)
    0x0026 00038 (main.go:60)    LEAQ    800(SP), BP
    0x002e 00046 (main.go:60)    FUNCDATA    $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
    0x002e 00046 (main.go:60)    FUNCDATA    $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
    0x002e 00046 (main.go:60)    FUNCDATA    $3, gclocals·39825eea4be6e41a70480a53a624f97b(SB)
    0x002e 00046 (main.go:62)    PCDATA    $2, $1
    0x002e 00046 (main.go:62)    PCDATA    $0, $0
    0x002e 00046 (main.go:62)    LEAQ    ""..autotmp_3(SP), DI
    0x0032 00050 (main.go:62)    XORPS    X0, X0
    0x0035 00053 (main.go:62)    PCDATA    $2, $0
    0x0035 00053 (main.go:62)    LEAQ    -32(DI), DI
    0x0039 00057 (main.go:62)    DUFFZERO    $64
    0x004c 00076 (main.go:62)    XORL    AX, AX
    0x004e 00078 (main.go:62)    XORL    CX, CX
    0x0050 00080 (main.go:62)    JMP    92
    0x0052 00082 (main.go:62)    MOVQ    ""..autotmp_3(SP)(AX*8), DX
    0x0056 00086 (main.go:62)    INCQ    AX
    0x0059 00089 (main.go:63)    ADDQ    DX, CX
    0x005c 00092 (main.go:62)    CMPQ    AX, $100
    0x0060 00096 (main.go:62)    JLT    82
    0x0062 00098 (main.go:65)    MOVQ    CX, "".sum+816(SP)
    0x006a 00106 (main.go:65)    MOVQ    800(SP), BP
    0x0072 00114 (main.go:65)    ADDQ    $808, SP
    0x0079 00121 (main.go:65)    RET
    0x007a 00122 (main.go:65)    NOP
    0x007a 00122 (main.go:60)    PCDATA    $0, $-1
    0x007a 00122 (main.go:60)    PCDATA    $2, $-1
    0x007a 00122 (main.go:60)    CALL    runtime.morestack_noctxt(SB) //扩充stack
    0x007f 00127 (main.go:60)    JMP    0

具体调用链路为: morestack_noctxt=>runtime.morestack=>runtime·newstack

func newstack() {
     ......
   ......
    preempt := atomic.Loaduintptr(&gp.stackguard0) == stackPreempt
    
    // Be conservative about where we preempt.
    // We are interested in preempting user Go code, not runtime code.
    // If we're holding locks, mallocing, or preemption is disabled, don't
    // preempt.
    // This check is very early in newstack so that even the status change
    // from Grunning to Gwaiting and back doesn't happen in this case.
    // That status change by itself can be viewed as a small preemption,
    // because the GC might change Gwaiting to Gscanwaiting, and then
    // this goroutine has to wait for the GC to finish before continuing.
    // If the GC is in some way dependent on this goroutine (for example,
    // it needs a lock held by the goroutine), that small preemption turns
    // into a real deadlock.
    if preempt {
    //注意点.
        if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.preemptoff != "" || thisg.m.p.ptr().status != _Prunning {
            // Let the goroutine keep running for now.
            // gp->preempt is set, so it will be preempted next time.
            gp.stackguard0 = gp.stack.lo + _StackGuard
            gogo(&gp.sched) // never return
        }
    }

 ......
 ......
 ......

    if preempt {
        if gp == thisg.m.g0 {
            throw("runtime: preempt g0")
        }
        if thisg.m.p == 0 && thisg.m.locks == 0 {
            throw("runtime: g is running but p is not")
        }
        // Synchronize with scang.
        casgstatus(gp, _Grunning, _Gwaiting)
        if gp.preemptscan {
        ......
        }
        // Act like goroutine called runtime.Gosched.
        casgstatus(gp, _Gwaiting, _Grunning    )
        gopreempt_m(gp) // never return
    }
  ......
}

最终gopreempt_m来抢占当前的g,gopreempt_m最终调用的还是goschedImpl .

把当前的G状态改为runnable,并且放到全局G队列中.
调用schedule() 重新执行一次调度,来继续运行其它可运行的G.

func gopreempt_m(gp *g) {
    if trace.enabled {
        traceGoPreempt()
    }
    goschedImpl(gp)
}

func goschedImpl(gp *g) {
    status := readgstatus(gp)
    if status&^_Gscan != _Grunning {
        dumpgstatus(gp)
        throw("bad g status")
    }
    casgstatus(gp, _Grunning, _Grunnable)
    dropg()
    lock(&sched.lock)
    globrunqput(gp)
    unlock(&sched.lock)

    schedule()
}

三. 实验

3.1 测试程序

注意点:

确保User Code部分执行时长能够超过10ms,并且中间不能调用任何会导致切换的代码.
确保User Code 能够促发抢占让出逻辑,非内联函数调用,即call时候会调用newstack
无其余如网络IO等干扰因素.

package main

import (
    "runtime"
    "time"
    "flag"
)

func testfunc()(sum int){
    var nums[100] int
    for _, num := range nums {
        sum += num
    }
    return
}

func main() {

    var numOfG = 1

    runtime.GOMAXPROCS(2)
    flag.IntVar(&numOfG, "n", 1, "num of g")
    flag.Parse()
    for i:=0 ; i< numOfG ; i++{
        go func() {
            for{
                testfunc()
            }
        }()
    }

     time.Sleep(time.Second*100)
}

我们设置GOMAXPROCS=2,来更好的观察上面的分析结果, 并通过死循环模拟User Code部分执行过久的情况,采样间隔为1000ms.

3.2实验数据

3.2.1 MAXPROCS>死循环G个数

SCHED 0ms: gomaxprocs=8 idleprocs=6 threads=3 spinningthreads=1 idlethreads=0 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=0 syscalltick=0 m=2 runqsize=0 gfreecnt=0
  P2: status=0 schedtick=0 syscalltick=0 m=-1 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
  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=1 dying=0 spinning=false blocked=false lockedg=1
  G1: status=4(chan receive) m=-1 lockedm=0
  G2: status=1() m=-1 lockedm=-1
  G3: status=1() m=-1 lockedm=-1
SCHED 1003ms: gomaxprocs=2 idleprocs=1 threads=5 spinningthreads=0 idlethreads=2 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=2 syscalltick=6 m=-1 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=48 syscalltick=10 m=3 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=1 dying=0 spinning=false blocked=false 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=3() m=0 lockedm=-1
SCHED 2005ms: gomaxprocs=2 idleprocs=1 threads=5 spinningthreads=0 idlethreads=2 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=2 syscalltick=6 m=-1 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=91 syscalltick=10 m=3 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=1 dying=0 spinning=false blocked=false 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=3() m=0 lockedm=-1
SCHED 3011ms: gomaxprocs=2 idleprocs=1 threads=5 spinningthreads=0 idlethreads=2 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=2 syscalltick=6 m=-1 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=135 syscalltick=10 m=3 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=1 dying=0 spinning=false blocked=false 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=3() m=0 lockedm=-1
SCHED 4015ms: gomaxprocs=2 idleprocs=1 threads=5 spinningthreads=0 idlethreads=2 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=2 syscalltick=6 m=-1 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=179 syscalltick=10 m=3 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=4 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=2() m=3 lockedm=-1
  G5: status=3() m=0 lockedm=-1

Goroutine	Total		Execution	Network wait	Sync block	Blocking syscall	Scheduler wait	GC sweeping	GC pause
4	5002ms		4999ms	0ns	0ns	0ns	2485µs	0ns (0.0%)	0ns (0.0%

可以发现G4为死循环的G,在P0上执行,每一秒中新增的schedtick约为50, 也就是实际调度约为20ms一次.

3.2.2 MAXPROCS=死循环G个数

SCHED 0ms: gomaxprocs=8 idleprocs=6 threads=3 spinningthreads=1 idlethreads=0 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=0 schedtick=0 syscalltick=0 m=-1 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=0 syscalltick=0 m=2 runqsize=0 gfreecnt=0
  P2: status=0 schedtick=0 syscalltick=0 m=-1 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
  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=1 dying=0 spinning=false blocked=false lockedg=1
  G1: status=4(chan receive) m=-1 lockedm=0
  G2: status=1() m=-1 lockedm=-1
  G3: status=1() m=-1 lockedm=-1
SCHED 1006ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=47 syscalltick=6 m=2 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=49 syscalltick=8 m=3 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=5 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 curg=4 mallocing=0 throwing=0 preemptoff= locks=0 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=6 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=2() m=2 lockedm=-1
  G5: status=2() m=3 lockedm=-1
  G6: status=3() m=0 lockedm=-1
SCHED 2013ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=91 syscalltick=6 m=2 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=93 syscalltick=8 m=3 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=4 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 curg=5 mallocing=0 throwing=0 preemptoff= locks=0 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=6 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=2() m=3 lockedm=-1
  G5: status=2() m=2 lockedm=-1
  G6: status=3() m=0 lockedm=-1
SCHED 3018ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=135 syscalltick=6 m=2 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=137 syscalltick=8 m=3 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=4 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 curg=5 mallocing=0 throwing=0 preemptoff= locks=0 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=6 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=2() m=3 lockedm=-1
  G5: status=2() m=2 lockedm=-1
  G6: status=3() m=0 lockedm=-1
SCHED 4029ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=179 syscalltick=6 m=2 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=181 syscalltick=8 m=3 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=1 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 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=6 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=1() m=-1 lockedm=-1
  G6: status=3() m=0 lockedm=-1
SCHED 5038ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=2 runqueue=1 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=226 syscalltick=8 m=0 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=227 syscalltick=8 m=3 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=4 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false 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=0 curg=5 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  G1: status=4(semacquire) m=-1 lockedm=-1
  G2: status=4(force gc (idle)) m=-1 lockedm=-1
  G3: status=4(GC sweep wait) m=-1 lockedm=-1
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=1(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=2() m=3 lockedm=-1
  G5: status=2() m=0 lockedm=-1
  G6: status=4(timer goroutine (idle)) m=-1 lockedm=-1

G4/G5为死循环G, 与第一个实验数据类似.

Goroutine	Total		Execution	Network wait	Sync block	Blocking syscall	Scheduler wait	GC sweeping	GC pause
4	5021ms		5019ms	0ns	0ns	0ns	2156µs	0ns (0.0%)	0ns (0.0%)
5	5021ms		5018ms	0ns	0ns	0ns	2442µs	0ns (0.0%)	0ns (0.0%

3.2.3 MAXPROCS<死循环G个数

我们通过开启5个死循环G来实验.

SCHED 0ms: gomaxprocs=8 idleprocs=5 threads=5 spinningthreads=1 idlethreads=0 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=0 syscalltick=0 m=3 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=1 syscalltick=0 m=2 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=0 curg=-1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=1 curg=-1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 spinning=true 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=1 dying=0 spinning=false blocked=false lockedg=1
  G1: status=1(chan receive) m=-1 lockedm=0
  G2: status=4(force gc (idle)) m=-1 lockedm=-1
  G3: status=4(GC sweep wait) m=-1 lockedm=-1
SCHED 1000ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=3 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=48 syscalltick=9 m=2 runqsize=0 gfreecnt=0
  P1: status=1 schedtick=49 syscalltick=6 m=3 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=7 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 curg=4 mallocing=0 throwing=0 preemptoff= locks=0 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=9 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=2() m=2 lockedm=-1
  G5: status=1() m=-1 lockedm=-1
  G6: status=1() m=-1 lockedm=-1
  G7: status=2() m=3 lockedm=-1
  G8: status=1() m=-1 lockedm=-1
  G9: status=3() m=0 lockedm=-1
SCHED 2003ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=1 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=91 syscalltick=9 m=2 runqsize=1 gfreecnt=0
  P1: status=1 schedtick=92 syscalltick=6 m=3 runqsize=1 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=8 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 curg=7 mallocing=0 throwing=0 preemptoff= locks=0 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=9 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=1() m=-1 lockedm=-1
  G6: status=1() m=-1 lockedm=-1
  G7: status=2() m=2 lockedm=-1
  G8: status=2() m=3 lockedm=-1
  G9: status=3() m=0 lockedm=-1
SCHED 3013ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=134 syscalltick=9 m=2 runqsize=1 gfreecnt=0
  P1: status=1 schedtick=135 syscalltick=6 m=3 runqsize=2 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=1 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 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=9 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=1() m=-1 lockedm=-1
  G6: status=1() m=-1 lockedm=-1
  G7: status=1() m=-1 lockedm=-1
  G8: status=1() m=-1 lockedm=-1
  G9: status=3() m=0 lockedm=-1
SCHED 4017ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=1 runqueue=1 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=178 syscalltick=9 m=2 runqsize=1 gfreecnt=0
  P1: status=1 schedtick=179 syscalltick=6 m=3 runqsize=1 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=7 mallocing=0 throwing=0 preemptoff= locks=0 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 curg=6 mallocing=0 throwing=0 preemptoff= locks=0 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=9 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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=1() m=-1 lockedm=-1
  G6: status=2() m=2 lockedm=-1
  G7: status=2() m=3 lockedm=-1
  G8: status=1() m=-1 lockedm=-1
  G9: status=3() m=0 lockedm=-1
SCHED 5019ms: gomaxprocs=2 idleprocs=0 threads=5 spinningthreads=0 idlethreads=2 runqueue=1 gcwaiting=0 nmidlelocked=0 stopwait=0 sysmonwait=0
  P0: status=1 schedtick=220 syscalltick=9 m=2 runqsize=2 gfreecnt=0
  P1: status=1 schedtick=221 syscalltick=6 m=3 runqsize=1 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=1 dying=0 spinning=false blocked=false lockedg=-1
  M2: p=0 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=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
  G17: status=4(finalizer wait) m=-1 lockedm=-1
  G33: status=4(trace reader (blocked)) m=-1 lockedm=-1
  G4: status=1() m=-1 lockedm=-1
  G5: status=1() m=-1 lockedm=-1
  G6: status=1() m=-1 lockedm=-1
  G7: status=1() m=-1 lockedm=-1
  G8: status=1() m=-1 lockedm=-1
  G9: status=1() m=-1 lockedm=-1

当死循环G超过MAXPROCS 可以明显发现5个死循环的G在轮转切换(抢占, P关联), 并且可以明显观察到全局G队列的变化,执行时长比较均匀.

Goroutine	Total	Execution	Network wait	Sync block	Blocking syscall	Scheduler wait	GC sweeping	GC pause
4	5040ms	2173ms	0ns	0ns	0ns	2867ms	0ns (0.0%)	0ns (0.0%)
5	5040ms	1950ms	0ns	0ns	0ns	3090ms	0ns (0.0%)	0ns (0.0%)
6	5040ms	2042ms	0ns	0ns	0ns	2997ms	0ns (0.0%)	0ns (0.0%)
7	5040ms	1938ms	0ns	0ns	0ns	3102ms	0ns (0.0%)	0ns (0.0%)
8	5040ms	1971ms	0ns	0ns	0ns	3069ms	0ns (0.0%)	0ns (0.0%

四.总结

本问只是简单的分析,抢占还有其他条件限制,但抢占基本是针对User Code的,不针对runtime,具体可见学习资料.
待后续.

最后 ending…如有不足请指点，亦可留言或联系 fobcrackgp@163.com.
本文为笃行原创文章首发于大题小作,永久链接:Golang 抢占调度流程分析

https://www.ifobnn.com/golangpreempt.html

有疑问加站长微信联系（非本文作者）

本文来自：Segmentfault

感谢作者：我不爱学习

查看原文：Golang 抢占调度流程分析

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Golang 抢占调度流程分析

Golang 抢占调度流程分析

一.前置知识

二. 源码分析总

2.1 总体框架

2.2 标记待抢占G

2.2.1 关键数据结构

2.2.2 扫描&标记

2.3 抢占触发

三. 实验

3.1 测试程序

3.2实验数据

3.2.1 MAXPROCS>死循环G个数

3.2.2 MAXPROCS=死循环G个数

3.2.3 MAXPROCS<死循环G个数

四.总结

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一周阅读排行

关注我

Golang 抢占调度流程分析

一.前置知识

二. 源码分析总

2.1 总体框架

2.2 标记待抢占G

2.2.1 关键数据结构

2.2.2 扫描&标记

2.3 抢占触发

三. 实验

3.1 测试程序

3.2实验数据

3.2.1 MAXPROCS>死循环G个数

3.2.2 MAXPROCS=死循环G个数

3.2.3 MAXPROCS<死循环G个数

四.总结

Golang 抢占调度流程分析

Golang 抢占调度流程分析

一.前置知识

二. 源码分析总

2.1 总体框架

2.2 标记待抢占G

2.2.1 关键数据结构

2.2.2 扫描&标记

2.3 抢占触发

三. 实验

3.1 测试程序

3.2实验数据

3.2.1 MAXPROCS>死循环G个数

3.2.2 MAXPROCS=死循环G个数

3.2.3 MAXPROCS<死循环G个数

四.总结

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一周阅读排行

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Golang 抢占调度流程分析

一.前置知识

二. 源码分析总

2.1 总体框架

2.2 标记待抢占G

2.2.1 关键数据结构

2.2.2 扫描&标记

2.3 抢占触发

三. 实验

3.1 测试程序

3.2实验数据

3.2.1 MAXPROCS>死循环G个数

3.2.2 MAXPROCS=死循环G个数

3.2.3 MAXPROCS<死循环G个数

四.总结

给该专栏投稿写篇新文章

收入到我管理的专栏新建专栏