perf: fps_max adjustments

backport fps_max 49 limit from CSGO

add a new frame limiter method which sleeps until a certain accuracy threshold
then, it will tightly loop while waiting for our yield to reach the end time

this ensures that we always arrive at the exact time that the FPS limit expects

before, this was sensitive to 2 issues, one being that the pause instruction
has different cycle counts on different CPUs and the other being that the
frame limit was not being handled efficiently within the busy wait period

Author: mastercoms

src/engine/sys_engine.cpp

@@ -35,6 +35,9 @@
 #include "vgui_baseui_interface.h"
 #endif
 #include "tier0/etwprof.h"
+#ifdef IS_WINDOWS_PC
+#include <windows.h>
+#endif
 
 // memdbgon must be the last include file in a .cpp file!!!
 #include "tier0/memdbgon.h"
@@ -244,13 +247,13 @@ bool CEngine::FilterTime( float dt )
 	// Dedicated's tic_rate regulates server frame rate.  Don't apply fps filter here.
 	// Only do this restriction on the client. Prevents clients from accomplishing certain
 	// hacks by pausing their client for a period of time.
-	if ( IsPC() && !sv.IsDedicated() && !CanCheat() && fps_max.GetFloat() < 30 )
+	if ( IsPC() && !sv.IsDedicated() && !CanCheat() && fps_max.GetFloat() < 49 )
 	{
 		// Don't do anything if fps_max=0 (which means it's unlimited).
 		if ( fps_max.GetFloat() != 0.0f )
 		{
-			Warning( "sv_cheats is 0 and fps_max is being limited to a minimum of 30 (or set to 0).\n" );
-			fps_max.SetValue( 30.0f );
+			Warning( "sv_cheats is 0 and fps_max is being limited to a minimum of 49 (or set to 0).\n" );
+			fps_max.SetValue( 49.0f );
 		}
 	}
 
@@ -343,7 +346,9 @@ void CEngine::Frame( void )
 		{
 			// ThreadSleep may be imprecise. On non-dedicated servers, we busy-sleep
 			// for the last one or two milliseconds to ensure very tight timing.
-			float fBusyWaitMS = IsWindows() ? 2.25f : 1.5f;
+			float fBusyWaitMS = IsWindows() ? 2.0f : 1.5f;
+			float fWaitTime = m_flMinFrameTime - m_flFrameTime;
+			float fWaitEnd = m_flCurrentTime + fWaitTime;
 			if ( sv.IsDedicated() )
 			{
 				fBusyWaitMS = host_timer_spin_ms.GetFloat();
@@ -354,23 +359,20 @@ void CEngine::Frame( void )
 			// to avoid wasting power and to let other threads/processes run.
 			// Calculate how long we need to wait.
 			int nSleepMS = (int)( ( m_flMinFrameTime - m_flFrameTime ) * 1000 - fBusyWaitMS );
-			if ( nSleepMS > 0 )
+			if ( nSleepMS > fBusyWaitMS )
 			{
 				ThreadSleep( nSleepMS );
 			}
-			else
+
+			while ( Plat_FloatTime() < fWaitEnd )
 			{
-				// On x86, busy-wait using PAUSE instruction which encourages
-				// power savings by idling for ~10 cycles (also yielding to
-				// the other logical hyperthread core if the CPU supports it)
-				for (int i = 2000; i >= 0; --i)
-				{
-#if defined(POSIX)
-					__asm( "pause" ); __asm( "pause" ); __asm( "pause" ); __asm( "pause" );
-#elif defined(IS_WINDOWS_PC)
-					_asm { pause }; _asm { pause }; _asm { pause }; _asm { pause };
+				ThreadPause();
+				// Yield the CPU to other threads.
+#ifdef IS_WINDOWS_PC
+				SwitchToThread();
+#elif defined( POSIX )
+				sched_yield();
 #endif
-				}
 			}
 
 			// Go back to the top of the loop and see if it is time yet.