为什么select {} vs select {case}导致不同的调度

Question

1.select {case}

    "fmt"
    "runtime"
    "time"
)

func main() {
    runtime.GOMAXPROCS(1)

    go func() {
        fmt.Println("a")
    }()
    go func() {
        fmt.Println("b")
    }()
    go func() {
        fmt.Println("c")
    }()

    fmt.Println("d")

    select {
    case _ = <-time.After(time.Second):
    }
}

result:dabc

2.select {}

package main
import (
    "fmt"
    "runtime"

)
func main() {
    runtime.GOMAXPROCS(1)

    go func() {
        fmt.Printf("a")

    }()

    go func() {
        fmt.Printf("b")

    }()

    go func() {
        fmt.Printf("c")

    }()

    fmt.Printf("d")
    select {}

}

result:dcab

Because I set "runtime.GOMAXPROCS(1)", there's only one "Processor" in my program.

One "Processor" has only one "runq", and in/out operation is "FIFO"

My question is "why always... not rand...."

Answer 1

Finally figured it out.

Processor.runq is FIFO, there's another variable(runnext) here, runnext saves the lastest goroutine which where will be processed next.

The latest goroutine kicks p.runnext

Timer creates a goroutine ,and this goroutine kicks p.runnext.

---

Processor的runnext 被定时器创建的goroutine kick调, 导致下一个执行的是定时器的goroutine,然后其他的FIFO