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101 lines
3.1 KiB
Go
101 lines
3.1 KiB
Go
// Copyright (C) 2018 The Syncthing Authors.
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//
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// This Source Code Form is subject to the terms of the Mozilla Public
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// License, v. 2.0. If a copy of the MPL was not distributed with this file,
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// You can obtain one at https://mozilla.org/MPL/2.0/.
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// +build !android
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package osutil
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import (
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"os"
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"syscall"
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"github.com/pkg/errors"
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)
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const ioprioClassShift = 13
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type ioprioClass int
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const (
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ioprioClassRT ioprioClass = iota + 1
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ioprioClassBE
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ioprioClassIdle
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)
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const (
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ioprioWhoProcess = iota + 1
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ioprioWhoPGRP
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ioprioWhoUser
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)
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func ioprioSet(class ioprioClass, value int) error {
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res, _, err := syscall.Syscall(syscall.SYS_IOPRIO_SET,
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uintptr(ioprioWhoProcess), 0,
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uintptr(class)<<ioprioClassShift|uintptr(value))
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if res == 0 {
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return nil
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}
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return err
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}
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// SetLowPriority lowers the process CPU scheduling priority, and possibly
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// I/O priority depending on the platform and OS.
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func SetLowPriority() error {
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// Process zero is "self", niceness value 9 is something between 0
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// (default) and 19 (worst priority). But then, this is Linux, so of
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// course we get this to take care of as well:
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//
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// "C library/kernel differences
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//
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// Within the kernel, nice values are actually represented using the
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// range 40..1 (since negative numbers are error codes) and these are
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// the values employed by the setpriority() and getpriority() system
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// calls. The glibc wrapper functions for these system calls handle the
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// translations between the user-land and kernel representations of the
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// nice value according to the formula unice = 20 - knice. (Thus, the
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// kernel's 40..1 range corresponds to the range -20..19 as seen by user
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// space.)"
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const (
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pidSelf = 0
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wantNiceLevel = 20 - 9
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)
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// Remember Linux kernel nice levels are upside down.
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if cur, err := syscall.Getpriority(syscall.PRIO_PROCESS, 0); err == nil && cur <= wantNiceLevel {
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// We're done here.
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return nil
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}
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// Move ourselves to a new process group so that we can use the process
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// group variants of Setpriority etc to affect all of our threads in one
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// go. If this fails, bail, so that we don't affect things we shouldn't.
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// If we are already the leader of our own process group, do nothing.
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//
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// Oh and this is because Linux doesn't follow the POSIX threading model
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// where setting the niceness of the process would actually set the
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// niceness of the process, instead it just affects the current thread
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// so we need this workaround...
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if pgid, err := syscall.Getpgid(pidSelf); err != nil {
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// This error really shouldn't happen
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return errors.Wrap(err, "get process group")
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} else if pgid != os.Getpid() {
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// We are not process group leader. Elevate!
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if err := syscall.Setpgid(pidSelf, 0); err != nil {
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return errors.Wrap(err, "set process group")
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}
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}
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if err := syscall.Setpriority(syscall.PRIO_PGRP, pidSelf, wantNiceLevel); err != nil {
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return errors.Wrap(err, "set niceness")
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}
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// Best effort, somewhere to the end of the scale (0 through 7 being the
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// range).
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err := ioprioSet(ioprioClassBE, 5)
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return errors.Wrap(err, "set I/O priority") // wraps nil as nil
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}
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