Main, OutputThread: increase kernel timer slack (Linux)

Allows the kernel to combine timer wakeups with other processes,
reducing power usage.

src/thread/Slack.hxx

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_THREAD_SLACK_HXX
+#define MPD_THREAD_SLACK_HXX
+
+#ifdef HAVE_PRCTL
+#include <sys/prctl.h>
+#endif
+
+/**
+ * Set the current thread's timer slack to the specified number of
+ * nanoseconds (requires Linux 2.6.28).  This allows the kernel to
+ * merge multiple wakeups, which is a trick to save energy.
+ */
+static inline void
+SetThreadTimerSlackNS(unsigned long slack_ns)
+{
+#if defined(HAVE_PRCTL) && defined(PR_SET_TIMERSLACK)
+	prctl(PR_SET_TIMERSLACK, slack_ns, 0, 0, 0);
+#else
+	(void)slack_ns;
+#endif
+}
+
+static inline void
+SetThreadTimerSlackUS(unsigned long slack_us)
+{
+	SetThreadTimerSlackNS(slack_us * 1000ul);
+}
+
+static inline void
+SetThreadTimerSlackMS(unsigned long slack_ms)
+{
+	SetThreadTimerSlackNS(slack_ms * 1000000ul);
+}
+
+#endif