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