golang buf_mmap 代码
文件路径：/src/cmd/internal/bio/buf_mmap.go
/ Copyright 2019 The Go Authors. All rights reserved.
/ Use of this source code is governed by a BSD-style
/ license that can be found in the LICENSE file.
/go:build darwin || dragonfly || freebsd || linux || netbsd || openbsd
/ +build darwin dragonfly freebsd linux netbsd openbsd
package bio
import (
"runtime"
"sync/atomic"
"syscall"
)
/ mmapLimit is the maximum number of mmaped regions to create before
/ falling back to reading into a heap-allocated slice. This exists
/ because some operating systems place a limit on the number of
/ distinct mapped regions per process. As of this writing:
/
/ Darwin unlimited
/ DragonFly 1000000 (vm.max_proc_mmap)
/ FreeBSD unlimited
/ Linux 65530 (vm.max_map_count) / TODO: query /proc/sys/vm/max_map_count?
/ NetBSD unlimited
/ OpenBSD unlimited
var mmapLimit int32 = 1<<31 - 1
func init() {
/ Linux is the only practically concerning OS.
if runtime.GOOS == "linux" {
mmapLimit = 30000
}
}
func (r *Reader) sliceOS(length uint64) ([]byte, bool) {
/ For small slices, don't bother with the overhead of a
/ mapping, especially since we have no way to unmap it.
const threshold = 16 << 10
if length < threshold {
return nil, false
}
/ Have we reached the mmap limit?
if atomic.AddInt32(&mmapLimit, -1) < 0 {
atomic.AddInt32(&mmapLimit, 1)
return nil, false
}
/ Page-align the offset.
off := r.Offset()
align := syscall.Getpagesize()
aoff := off &^ int64(align-1)
data, err := syscall.Mmap(int(r.f.Fd()), aoff, int(length+uint64(off-aoff)), syscall.PROT_READ, syscall.MAP_SHARED|syscall.MAP_FILE)
if err != nil {
return nil, false
}
data = data[off-aoff:]
r.MustSeek(int64(length), 1)
return data, true
}