content: Allow multiple raster threads on Android.

content/browser/gpu/compositor_util.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/gpu/compositor_util.h"
 
 #include <stddef.h>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 159 matching lines @@
 }
 
 }  // namespace
 
 int NumberOfRendererRasterThreads() {
   int num_processors = base::SysInfo::NumberOfProcessors();
 
 #if defined(OS_ANDROID)
   // Android may report 6 to 8 CPUs for big.LITTLE configurations.
   // Limit the number of raster threads based on maximum of 4 big cores.
   num_processors = std::min(num_processors, 4);

[aelias_OOO_until_Jul13, 2016/02/26 03:12:03]

How about moving this to the base::SysInfo::NumberOfProcessors() implementation?
 I think it's reasonable to define that to refer to the number of big cores. 
Every use case of the core count information I can think of is similar to this
one, to balance workloads and avoid oversaturation.

Ideally we would also use a system API that allows us to distinguish the cores,
instead of capping to the magic number 4 (but I'm not sure if anybody bothered
to provide that yet).

 #endif
 
   int num_raster_threads = num_processors / 2;
 
-#if defined(OS_ANDROID)
-  // Limit the number of raster threads to 1 on Android.
-  // TODO(reveman): Remove this when we have a better mechanims to prevent
-  // pre-paint raster work from slowing down non-raster work. crbug.com/504515
-  num_raster_threads = 1;

[aelias_OOO_until_Jul13, 2016/02/26 03:12:02]

vmpstr@ and ericrk@ mentioned today they'd also prefer to hold off on increasing
the number of foreground threads on Android until their image decode work is
further along.  I'm open to revisiting this later when we have more confidence
in our task policy.

As long as the code that feeds tasks into the raster system is entirely sane, I
have no real grounds to object on raising the number of threads here, I'm just
not sure we're at that point quite yet.

-#endif
-
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
 
   if (command_line.HasSwitch(switches::kNumRasterThreads)) {
     std::string string_value = command_line.GetSwitchValueASCII(
         switches::kNumRasterThreads);
     if (!base::StringToInt(string_value, &num_raster_threads)) {
       DLOG(WARNING) << "Failed to parse switch " <<
           switches::kNumRasterThreads  << ": " << string_value;
     }
@@ skipping 197 matching lines @@
     }
   }
   return problem_list;
 }
 
 std::vector<std::string> GetDriverBugWorkarounds() {
   return GpuDataManagerImpl::GetInstance()->GetDriverBugWorkarounds();
 }
 
 }  // namespace content