open/read vs open/mmap vs open/bufio.read on golang. syscall.Madvise() is unsupported darwin.

open_mmap_adv.go

package main

import (
	"fmt"
	"log"
	"os"
	"syscall"
)

func main() {
	if len(os.Args) < 2 {
		return
	}
	path := os.Args[1]

	f, err := os.Open(path)
	if err != nil {
		log.Fatal(err)
	}
	fi, err := f.Stat()
	if err != nil {
		log.Fatal(err)
	}

	mem, err := syscall.Mmap(int(f.Fd()), 0, int(fi.Size()),
							syscall.PROT_READ, syscall.MAP_SHARED)
	if err != nil {
		log.Fatal(err)
	}

	err = syscall.Madvise(mem, syscall.MADV_SEQUENTIAL | syscall.MADV_WILLNEED)
	if err != nil {
		log.Fatal(err)
	}

	cnt := 0
	for _, b := range mem {
		if b == 0x00 {
			cnt++
		}
	}
	err = syscall.Munmap(mem)
	if err != nil {
		log.Fatal(err)
	}
	f.Close()

	fmt.Printf("%s: size=%d cnt=%d\n", path, fi.Size(), cnt);
}