| // 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 unix | |
| 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 | |
| } | |