Throttle SetEvent call in render thread (#3511)

Reduce unnecessary SetEvent CPU overhead by using `std::atomic_flag` to avoid setting kernel events when the painting thread is already awake and running.

src/renderer/base/thread.cpp

@@ -17,6 +17,8 @@ RenderThread::RenderThread() :
     _fKeepRunning(true),
     _hPaintEnabledEvent(nullptr)
 {
+    _fNextFrameRequested.clear();
+    _fPainting.clear();
 }
 
 RenderThread::~RenderThread()
@@ -158,10 +160,21 @@ DWORD WINAPI RenderThread::_ThreadProc()
     while (_fKeepRunning)
     {
         WaitForSingleObject(_hPaintEnabledEvent, INFINITE);
-        WaitForSingleObject(_hEvent, INFINITE);
+
+        // Skip waiting if next frame is requested.
+        if (_fNextFrameRequested.test_and_set(std::memory_order_relaxed))
+        {
+            _fNextFrameRequested.clear(std::memory_order_relaxed);
+        }
+        else
+        {
+            WaitForSingleObject(_hEvent, INFINITE);
+        }
 
         ResetEvent(_hPaintCompletedEvent);
 
+        _fPainting.test_and_set(std::memory_order_acquire);
+
         LOG_IF_FAILED(_pRenderer->PaintFrame());
 
         SetEvent(_hPaintCompletedEvent);
@@ -171,6 +184,8 @@ DWORD WINAPI RenderThread::_ThreadProc()
         {
             Sleep(s_FrameLimitMilliseconds);
         }
+
+        _fPainting.clear(std::memory_order_release);
     }
 
     return S_OK;
@@ -178,6 +193,14 @@ DWORD WINAPI RenderThread::_ThreadProc()
 
 void RenderThread::NotifyPaint()
 {
+    // If we are currently painting a frame, set _fNextFrameRequested flag
+    // to indicate we want to paint next frame immediately.
+    if (_fPainting.test_and_set(std::memory_order_acquire))
+    {
+        _fNextFrameRequested.test_and_set(std::memory_order_relaxed);
+        return;
+    }
+
     SetEvent(_hEvent);
 }
 

src/renderer/base/thread.hpp

@@ -47,5 +47,7 @@ namespace Microsoft::Console::Render
         IRenderer* _pRenderer; // Non-ownership pointer
 
         bool _fKeepRunning;
+        std::atomic_flag _fNextFrameRequested;
+        std::atomic_flag _fPainting;
     };
 }