Quick & Dirty implementation of fincore in go

fincore_raw.go

package main

import (
	"fmt"
	"os"
	"syscall"
	"unsafe"
)

func fincore(path string) {
	file, err := os.Open(path)
	if err != nil {
		panic(err)
	}
	defer file.Close()

	fileInfo, err := file.Stat()
	if err != nil {
		panic(err)
	}

	fileOffset := int64(0)
	fileSize := int(fileInfo.Size())

	mmap, err := syscall.Mmap(int(file.Fd()), fileOffset, fileSize, syscall.PROT_NONE, syscall.MAP_SHARED)
	if err != nil {
		panic(err)
	}

	defer syscall.Munmap(mmap)

	vecsz := (int64(fileSize) + int64(os.Getpagesize()) - 1) / int64(os.Getpagesize())
	vec := make([]byte, vecsz)
	mmap_ptr := uintptr(unsafe.Pointer(&mmap[0]))
	size_ptr := uintptr(fileSize)
	vec_ptr := uintptr(unsafe.Pointer(&vec[0]))

	r1, _, err := syscall.Syscall(syscall.SYS_MINCORE, mmap_ptr, size_ptr, vec_ptr)
	if r1 != 0 {
		panic("Oh god, mincore failed.")
	}

	cachedPages := 0

	for _, page := range vec {
		if page&1 != 0 {
			cachedPages++
		}
	}

	fmt.Printf("%s: %d of %d pages cached\n", path, cachedPages, len(vec))

}

func main() {
	for _, location := range os.Args[1:] {
		fincore(location)
	}
}