Re-land: content: Use NumberOfProcessors() / 2 raster threads.

content/browser/gpu/compositor_util.cc

Index: content/browser/gpu/compositor_util.cc
diff --git a/content/browser/gpu/compositor_util.cc b/content/browser/gpu/compositor_util.cc
index a96cdeae4138e43adcfdccc3760e963dc2cce6a9..55bb0646ac5b4a52245807064263ed7623ed9a93 100644
--- a/content/browser/gpu/compositor_util.cc
+++ b/content/browser/gpu/compositor_util.cc
@@ -10,6 +10,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/sys_info.h"
 #include "build/build_config.h"
+#include "cc/base/math_util.h"
 #include "cc/base/switches.h"
 #include "content/browser/gpu/gpu_data_manager_impl.h"
 #include "content/public/common/content_switches.h"
@@ -210,14 +211,17 @@ bool IsImplSidePaintingEnabled() {
 }
 
 int NumberOfRendererRasterThreads() {
-  int num_raster_threads = 1;
-
-  // Async uploads uses its own thread, so allow an extra thread when async
-  // uploads is not in use.
-  bool allow_extra_thread =
+  int num_raster_threads = cc::MathUtil::ClampToRange(
+      base::SysInfo::NumberOfProcessors() / 2,
+      kMinRasterThreads, kMaxRasterThreads);

[danakj, 2015/05/13 20:01:23]

nit: how about do the computation for a number here and below without clamping,
then clamp at the end once.

[reveman, 2015/05/13 20:24:35]

I did that first but didn't like it as it's already clamped when
force_num_raster_threads is true. I guess I can refactor
ForceNumberOfRendererRasterThreads a bit to make more sense.

+
+  // Async uploads is used when neither zero-copy nor one-copy is enabled and
+  // it uses its own thread, so reduce the number of raster threads when async
+  // uploads is in use.
+  bool async_uploads_is_used =
       IsZeroCopyUploadEnabled() || IsOneCopyUploadEnabled();
-  if (base::SysInfo::NumberOfProcessors() >= 4 && allow_extra_thread)
-    num_raster_threads = 2;
+  if (async_uploads_is_used)
+    num_raster_threads = std::max(kMinRasterThreads, num_raster_threads - 1);
 
   int force_num_raster_threads = ForceNumberOfRendererRasterThreads();
   if (force_num_raster_threads)