Main, OutputThread: increase kernel timer slack (Linux)

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

NEWS

@@ -21,6 +21,7 @@ ver 0.19 (not yet released)
 * threads:
   - the update thread runs at "idle" priority
   - the output thread runs at "real-time" priority
+  - increase kernel timer slack on Linux
   - name each thread (for debugging)
 * new resampler option using libsoxr
 

configure.ac

@@ -184,6 +184,8 @@ AC_SEARCH_LIBS([exp], [m],,
 AC_CHECK_HEADERS(locale.h)
 AC_CHECK_HEADERS(valgrind/memcheck.h)
 
+AC_CHECK_HEADERS([sys/prctl.h], AC_CHECK_FUNCS([prctl]))
+
 AC_CHECK_LIB([pthread], [pthread_setname_np],
 	[have_pthread_setname_np=yes],
 	[have_pthread_setname_np=no])

src/Main.cxx

@@ -60,6 +60,7 @@
 #include "util/Error.hxx"
 #include "util/Domain.hxx"
 #include "thread/Id.hxx"
+#include "thread/Slack.hxx"
 #include "config/ConfigGlobal.hxx"
 #include "config/ConfigData.hxx"
 #include "config/ConfigDefaults.hxx"
@@ -523,6 +524,10 @@ int mpd_main(int argc, char *argv[])
 	win32_app_started();
 #endif
 
+	/* the MPD frontend does not care about timer slack; set it to
+	   a huge value to allow the kernel to reduce CPU wakeups */
+	SetThreadTimerSlackMS(100);
+
 	/* run the main loop */
 	main_loop->Run();
 

src/output/OutputThread.cxx

@@ -30,6 +30,7 @@
 #include "MusicPipe.hxx"
 #include "MusicChunk.hxx"
 #include "thread/Util.hxx"
+#include "thread/Slack.hxx"
 #include "thread/Name.hxx"
 #include "system/FatalError.hxx"
 #include "util/Error.hxx"
@@ -561,6 +562,7 @@ AudioOutput::Task()
 	FormatThreadName("output:%s", name);
 
 	SetThreadRealtime();
+	SetThreadTimerSlackUS(100);
 
 	mutex.lock();
 

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