thread/Util: lower the real-time priority from 50 to 40

On linux-rt, kernel IRQ threads are configured with priority=50, and
this change configures MPD somewhat below that priority, leaving some
room for other programs to be configured in between.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/643

NEWS

@@ -28,6 +28,7 @@ ver 0.22 (not yet released)
   - jack: add option "auto_destination_ports"
   - jack: report error details
   - pulse: add option "media_role"
+* lower the real-time priority from 50 to 40
 * switch to C++17
   - GCC 7 or clang 4 (or newer) recommended
 

doc/user.rst

@@ -804,13 +804,13 @@ This is only possible you allow :program:`MPD` to do it. This privilege is contr
 
 .. code-block:: none
 
-    ulimit -HS -r 50; mpd
+    ulimit -HS -r 40; mpd
 
 Or you can use the :command:`prlimit` program from the util-linux package:
 
 .. code-block:: none
 
-    prlimit --rtprio=50 --rttime=unlimited mpd
+    prlimit --rtprio=40 --rttime=unlimited mpd
 
 The systemd service file shipped with :program:`MPD` comes with this setting.
 
@@ -828,10 +828,10 @@ You can verify whether the real-time scheduler is active with the ps command:
       PID   TID CLS RTPRIO COMMAND
     16257 16257  TS      - mpd
     16257 16258  TS      - io
-    16257 16259  FF     50 rtio
+    16257 16259  FF     40 rtio
     16257 16260  TS      - player
     16257 16261  TS      - decoder
-    16257 16262  FF     50 output:ALSA
+    16257 16262  FF     40 output:ALSA
     16257 16263 IDL      0 update
 
 The CLS column shows the CPU scheduler; TS is the normal scheduler; FF and RR are real-time schedulers. In this example, two threads use the real-time scheduler: the output thread and the rtio (real-time I/O) thread; these two are the important ones. The database update thread uses the idle scheduler ("IDL in ps), which only gets CPU when no other process needs it.

src/thread/Util.cxx

@@ -101,7 +101,7 @@ SetThreadRealtime()
 {
 #ifdef __linux__
 	struct sched_param sched_param;
-	sched_param.sched_priority = 50;
+	sched_param.sched_priority = 40;
 
 	int policy = SCHED_FIFO;
 #ifdef SCHED_RESET_ON_FORK

systemd/system/mpd.service.in

@@ -12,8 +12,8 @@ ExecStart=@prefix@/bin/mpd --no-daemon
 # periodic wakeups which are unnecessary if MPD is not playing.
 #WatchdogSec=120
 
-# allow MPD to use real-time priority 50
-LimitRTPRIO=50
+# allow MPD to use real-time priority 40
+LimitRTPRIO=40
 LimitRTTIME=infinity
 
 # disallow writing to /usr, /bin, /sbin, ...

systemd/user/mpd.service.in

@@ -12,8 +12,8 @@ ExecStart=@prefix@/bin/mpd --no-daemon
 # periodic wakeups which are unnecessary if MPD is not playing.
 #WatchdogSec=120
 
-# allow MPD to use real-time priority 50
-LimitRTPRIO=50
+# allow MPD to use real-time priority 40
+LimitRTPRIO=40
 LimitRTTIME=infinity
 
 # disallow writing to /usr, /bin, /sbin, ...