golang一个P最多能放几个g?

先说结论：257个！

首先介绍下时间片

一个p最多持续运行10ms，就要执行下一次调度（由go监控线程执行）。

相关代码段

路径：/go/src/runtime/proc.go

// 10ms
const forcePreemptNS = 10 * 1000 * 1000

路径：/go/src/runtime/proc.go line:5503

当p的上次调度时间+10ms <= 当前时间，说明这个g运行太久了，该换换了。

if pd.schedwhen+forcePreemptNS <= now {
    preemptone(_p_) // 执行goroutine抢占

    // In case of syscall, preemptone() doesn't

    // work, because there is no M wired to P.
    sysretake = true
}

再看P在golang中的声明

p中runq字段是一个数组存放着多个goroutine,，可存放256个。

p中runnext字段可以存放一个goroutine。

runnext字段的大致作用，一个P如果还有剩余时间片，那么P中next字段中的goroutine会继承这部分剩余时间并执行。

路径：go/src/runtime/runtime2.go line:604

type p struct {
   ...忽略部分...
   runq     [256]guintptr
   // runnext, if non-nil, is a runnable G that was ready'd by
   // the current G and should be run next instead of what's in
   // runq if there's time remaining in the running G's time
   // slice. It will inherit the time left in the current time
   // slice. If a set of goroutines is locked in a
   // communicate-and-wait pattern, this schedules that set as a
   // unit and eliminates the (potentially large) scheduling
   // latency that otherwise arises from adding the ready'd
   // goroutines to the end of the run queue.
   //
   // Note that while other P's may atomically CAS this to zero,
   // only the owner P can CAS it to a valid G.
   runnext guintptr

“幼麟yysd”