SetReadBuffer effect on UDP listener

gistfile1.txt

$ cat > udpbufcap.go << EOF
package main

import (
        "fmt"
        "io/ioutil"
        "net"
        "os"
        "strconv"
        "strings"
)

func main() {
        // Listen on UDP.
        receiver, err := net.ListenUDP("udp", nil)
        if err != nil {
                panic(err)
        }

        // (maybe) Bump receive buffer.
        switch os.Args[1] {
        case "base":
                fmt.Printf("keep rcvbuf as is\n")
        case "test":
                rcvbufMax := readint("/proc/sys/net/core/rmem_max")
                fmt.Printf("set rcvbuf to %d\n", rcvbufMax)
                if err := receiver.SetReadBuffer(rcvbufMax); err != nil {
                        panic(err)
                }
        }

        // Send many unique packets.
        sender, err := net.Dial("udp", receiver.LocalAddr().String())
        if err != nil {
                panic(err)
        }
        buf := make([]byte, 1024)
        for i := 0; i < 10000; i++ {
                buf = append(buf[:0], fmt.Sprintf("seq=%08d!", i)...)
                buf = buf[:cap(buf)]
                if _, err := sender.Write(buf); err != nil {
                        panic(err)
                }
        }

        // Read back as many packets as possible, printing identity of each.
        buf = make([]byte, cap(buf))
        for i := 0; i < 10000; i++ {
                n, err := receiver.Read(buf)
                if err != nil {
                        panic(err)
                }
                if n != cap(buf) {
                        panic(n)
                }
                fmt.Printf("i=%5d buf[:16]=%.16q\n", i, buf)
        }
}

func readint(path string) int {
        buf, err := ioutil.ReadFile(path)
        if err != nil {
                panic(err)
        }
        n, err := strconv.ParseInt(strings.TrimSpace(string(buf)), 10, 64)
        if err != nil {
                panic(err)
        }
        return int(n)
}
EOF
$ go build udpbufcap.go
$ ./udpbufcap base
keep rcvbuf as is
i=    0 buf[:16]="seq=00000000!\x00\x00\x00"
i=    1 buf[:16]="seq=00000001!\x00\x00\x00"
i=    2 buf[:16]="seq=00000002!\x00\x00\x00"
(...)
i=   90 buf[:16]="seq=00000090!\x00\x00\x00"
i=   91 buf[:16]="seq=00000091!\x00\x00\x00"
i=   92 buf[:16]="seq=00000092!\x00\x00\x00"
$ ./udpbufcap test
set rcvbuf to 8738000
i=    0 buf[:16]="seq=00000000!\x00\x00\x00"
i=    1 buf[:16]="seq=00000001!\x00\x00\x00"
i=    2 buf[:16]="seq=00000002!\x00\x00\x00"
(...)
i= 7583 buf[:16]="seq=00007583!\x00\x00\x00"
i= 7584 buf[:16]="seq=00007584!\x00\x00\x00"
i= 7585 buf[:16]="seq=00007585!\x00\x00\x00"
$