Limit the number of WebGL contexts that are active at any given time

Source/core/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/core/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/core/platform/graphics/gpu/DrawingBuffer.cpp b/Source/core/platform/graphics/gpu/DrawingBuffer.cpp
index 33855a04085adb6d95aa2ea8dc6a223ff0b7cb64..64ea9a424d0341963ca99638bebf3697768280c8 100644
--- a/Source/core/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/core/platform/graphics/gpu/DrawingBuffer.cpp
@@ -39,6 +39,7 @@
 #include "GraphicsLayerChromium.h"
 #include "ImageData.h"
 #include "TraceEvent.h"
+#include "WebGLRenderingContext.h"

[bajones, 2013/04/20 05:19:59]

Ken already pointed out in a different patch review that this violates layering
rules, but I'm not sure what the appropriate approach is to allow this. Include
is currently required to provide access to
WebGLRenderingContext::forciblyLoseOldestContext();

 #include <algorithm>
 #include <public/Platform.h>
 #include <public/WebCompositorSupport.h>
@@ -58,6 +59,9 @@ static const int s_maximumResourceUsePixels = 16 * 1024 * 1024;
 static int s_currentResourceUsePixels = 0;
 static const float s_resourceAdjustedRatio = 0.5;
 
+static const bool s_allowContextEvictionOnCreate = true;
+static const int s_maxScaleAttempts = 3;
+
 class ScopedTextureUnit0BindingRestorer {
 public:
     ScopedTextureUnit0BindingRestorer(GraphicsContext3D* context, GC3Denum activeTextureUnit, Platform3DObject textureUnitZeroId)
@@ -577,6 +581,8 @@ void DrawingBuffer::resizeDepthStencil(const IntSize& size, int sampleCount)
     m_context->bindRenderbuffer(GraphicsContext3D::RENDERBUFFER, 0);
 }
 
+
+
 void DrawingBuffer::clearFramebuffers(GC3Dbitfield clearMask)
 {
     m_context->bindFramebuffer(GraphicsContext3D::FRAMEBUFFER, m_multisampleFBO ? m_multisampleFBO : m_fbo);
@@ -648,21 +654,56 @@ IntSize DrawingBuffer::adjustSize(const IntSize& size) {
         adjustedSize.setWidth(maxTextureSize);
 
     // Try progressively smaller sizes until we find a size that fits or reach a scale limit.
+    int scaleAttempts = 0;
     while ((s_currentResourceUsePixels + pixelDelta(adjustedSize)) > s_maximumResourceUsePixels) {
+        scaleAttempts++;
+        if(s_maxScaleAttempts > -1 && scaleAttempts > s_maxScaleAttempts)
+            return IntSize();
+
         adjustedSize.scale(s_resourceAdjustedRatio);
 
         if (adjustedSize.isEmpty())
             return IntSize();
     }
+
+    return adjustedSize;
+}
+
+IntSize DrawingBuffer::adjustSizeWithContextEviction(const IntSize& size, bool& evictContext) {
+    IntSize adjustedSize = adjustSize(size);
+    if(!adjustedSize.isEmpty()) {
+        evictContext = false;
+        return adjustedSize; // Buffer fits without evicting a context
+    }
+
+    // Speculatively adjust the pixel budget to see if the buffer would fit should the oldest context be evicted
+    IntSize oldestSize = WebGLRenderingContext::oldestContextSize();
+    int pixelDelta = oldestSize.width() * oldestSize.height();
+
+    s_currentResourceUsePixels -= pixelDelta;
+    adjustedSize = adjustSize(size);
+    s_currentResourceUsePixels += pixelDelta;
+
+    evictContext = !adjustedSize.isEmpty();
     return adjustedSize;
 }
 
 void DrawingBuffer::reset(const IntSize& newSize)
 {
-    IntSize adjustedSize = adjustSize(newSize);
+    IntSize adjustedSize;
+    bool evictContext = false;
+    bool isNewContext = m_size.isEmpty();
+    if (s_allowContextEvictionOnCreate && isNewContext)
+        adjustedSize = adjustSizeWithContextEviction(newSize, evictContext);
+    else
+        adjustSize(newSize);
+
     if (adjustedSize.isEmpty())
         return;
 
+    if (evictContext)
+        WebGLRenderingContext::forciblyLoseOldestContext();
+
     if (adjustedSize != m_size) {
         do {
             // resize multisample FBO