Optionally restore arbitrary sleep between frames in dedicated server

This was removed while introducing com_busyWait (PR JACoders#695). Before
merging PR JACoders#695, client behaviour was equivalent to com_busyWait 1,
while dedicated server behaviour was broadly similar to com_busyWait 0
(run when there is network I/O, or as much as is necessary for sv_fps).
However, dedicated servers also had a hard-coded 5ms sleep before each
frame during which they did nothing, not even processing network I/O,
resulting in the dedicated server never using as much as a full CPU
core per process even if the operating system scheduler would have
allowed it. That sleep was not reflected in the new code path for
com_busyWait 0.

This commit makes the sleep time configurable via a new cvar.
com_dedicated_sleep 0 is the same as PR JACoders#695 and ioquake3 (best
performance), while com_dedicated_sleep 5 is the same as historical
OpenJK behaviour (reduced performance, increased latency, ensures
that CPU time is made available to other processes). Larger values
result in greater reductions to both CPU usage and performance.

codemp/qcommon/common.cpp

@@ -41,6 +41,7 @@ fileHandle_t	com_journalDataFile;		// config files are written here
 cvar_t	*com_speeds;
 cvar_t	*com_developer;
 cvar_t	*com_dedicated;
+cvar_t	*com_dedicated_sleep;
 cvar_t	*com_timescale;
 cvar_t	*com_fixedtime;
 cvar_t	*com_journal;
@@ -1200,6 +1201,12 @@ void Com_Init( char *commandLine ) {
 		com_dedicated = Cvar_Get ("_dedicated", "0", CVAR_ROM|CVAR_INIT|CVAR_PROTECTED);
 	//	Cvar_CheckRange( com_dedicated, 0, 2, qtrue );
 	#endif
+
+		// If positive, the dedicated server will sleep this long
+		// before each frame, reducing CPU usage but increasing
+		// latency. This is basically cooperative multi-tasking.
+		com_dedicated_sleep = Cvar_Get("com_dedicated_sleep", "0", CVAR_ARCHIVE);
+
 		// allocate the stack based hunk allocator
 		Com_InitHunkMemory();
 

codemp/qcommon/qcommon.h

@@ -772,6 +772,7 @@ void		Com_StartupVariable( const char *match );
 
 extern	cvar_t	*com_developer;
 extern	cvar_t	*com_dedicated;
+extern	cvar_t	*com_dedicated_sleep;
 extern	cvar_t	*com_speeds;
 extern	cvar_t	*com_timescale;
 extern	cvar_t	*com_sv_running;

shared/sys/sys_main.cpp

@@ -768,26 +768,23 @@ int main ( int argc, char* argv[] )
 	// main game loop
 	while (1)
 	{
-		if ( com_busyWait->integer )
-		{
-			bool shouldSleep = false;
+		int yield_msec;
 
 #if !defined(_JK2EXE)
-			if ( com_dedicated->integer )
-			{
-				shouldSleep = true;
-			}
+		if ( com_dedicated->integer && com_dedicated_sleep->integer )
+		{
+			yield_msec = com_dedicated_sleep->integer;
+		}
 #endif
 
-			if ( com_minimized->integer )
-			{
-				shouldSleep = true;
-			}
+		if ( com_minimized->integer && com_busyWait->integer )
+		{
+			yield_msec = 5;
+		}
 
-			if ( shouldSleep )
-			{
-				Sys_Sleep( 5 );
-			}
+		if ( yield_msec > 0 )
+		{
+			Sys_Sleep( yield_msec );
 		}
 
 		// make sure mouse and joystick are only called once a frame