mincore in Go

mincore.go

package main

import (
    "flag"
    "log"
    "os"
    "syscall"
    "unsafe"
)

func main() {
    flag.Parse()
    for _, fname := range flag.Args() {
        Mincore(fname)
    }
}

func Mincore(fname string) {
    f, err := os.Open(fname)
    if err != nil {
        log.Fatalf("Could not open file '%s' for read: %s\n", fname, err)
    }
    defer f.Close()

    fi, err := f.Stat()
    if err != nil {
        log.Fatalf("Could not stat file %s: %s\n", fname, err)
    }
    if fi.Size() == 0 {
        log.Fatalf("%s appears to be 0 bytes in length\n", fname)
    }

    mmap, err := syscall.Mmap(int(f.Fd()), 0, int(fi.Size()), syscall.PROT_NONE, syscall.MAP_SHARED)
    if err != nil {
        log.Fatalf("Could not mmap file '%s': %s\n", fname, err)
    }
    defer syscall.Munmap(mmap)
    // TODO: check for MAP_FAILED which is ((void *) -1)
    // but maybe unnecessary since it looks like errno is always set when MAP_FAILED

    // one byte per page, only LSB is used, remainder is reserved and clear
    vecsz := (fi.Size() + int64(os.Getpagesize()) - 1) / int64(os.Getpagesize())
    vec := make([]byte, vecsz)

    mmap_ptr := uintptr(unsafe.Pointer(&mmap[0]))
    size_ptr := uintptr(fi.Size())
    vec_ptr := uintptr(unsafe.Pointer(&vec[0]))
    ret, _, err := syscall.RawSyscall(syscall.SYS_MINCORE, mmap_ptr, size_ptr, vec_ptr)
    if ret != 0 {
        log.Fatalf("mincore() failed: %s", err)
    }
    
    for i, b := range vec {
        if b % 2 == 1 {
            log.Printf("%d -> cached", i)
        } else {
            log.Printf("%d -> NOT", i)
        }
    }
}

strace.txt

open("main.go", O_RDONLY|O_CLOEXEC)     = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=1605, ...}) = 0
mmap(NULL, 1605, PROT_NONE, MAP_SHARED, 3, 0) = 0x7fd82e704000
mincore(0x7fd82e704000, 1605, [10000000000000000000000000000000...]) = 0
munmap(0x7fd82e704000, 1605)            = 0
close(3)                                = 0

test.txt

atobey@brak ~/src/mincore $ ./mincore main.go
2014/04/30 17:14:30 Cached pages: 1, Uncached: 0
atobey@brak ~/src/mincore $ echo 1 |sudo tee /proc/sys/vm/drop_caches
1
atobey@brak ~/src/mincore $ ./mincore main.go
2014/04/30 17:14:40 Cached pages: 0, Uncached: 1