cmd/syncthing, lib/config, lib/osutil: Lower process priority (fixes #4628)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4675

lib/osutil/lowprio_linux.go

@@ -0,0 +1,61 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package osutil
+
+import (
+	"syscall"
+
+	"github.com/pkg/errors"
+)
+
+const ioprioClassShift = 13
+
+type ioprioClass int
+
+const (
+	ioprioClassRT ioprioClass = iota + 1
+	ioprioClassBE
+	ioprioClassIdle
+)
+
+const (
+	ioprioWhoProcess = iota + 1
+	ioprioWhoPGRP
+	ioprioWhoUser
+)
+
+func ioprioSet(class ioprioClass, value int) error {
+	res, _, err := syscall.Syscall(syscall.SYS_IOPRIO_SET,
+		uintptr(ioprioWhoProcess), 0,
+		uintptr(class)<<ioprioClassShift|uintptr(value))
+	if res == 0 {
+		return nil
+	}
+	return err
+}
+
+// SetLowPriority lowers the process CPU scheduling priority, and possibly
+// I/O priority depending on the platform and OS.
+func SetLowPriority() error {
+	// Move ourselves to a new process group so that we can use the process
+	// group variants of Setpriority etc to affect all of our threads in one
+	// go. If this fails, bail, so that we don't affect things we shouldn't.
+	if err := syscall.Setpgid(0, 0); err != nil {
+		return errors.Wrap(err, "set process group")
+	}
+
+	// Process zero is "self", niceness value 9 is something between 0
+	// (default) and 19 (worst priority).
+	if err := syscall.Setpriority(syscall.PRIO_PGRP, 0, 9); err != nil {
+		return errors.Wrap(err, "set niceness")
+	}
+
+	// Best effort, somewhere to the end of the scale (0 through 7 being the
+	// range).
+	err := ioprioSet(ioprioClassBE, 5)
+	return errors.Wrap(err, "set I/O priority") // wraps nil as nil
+}