Use 2 buffers on presumable good Windows boxes.

media/audio/audio_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.
 
 // Software adjust volume of samples, allows each audio stream its own
 // volume without impacting master volume for chrome and other applications.
 
 // Implemented as templates to allow 8, 16 and 32 bit implementations.
 // 8 bit is unsigned and biased by 128.
 
 // TODO(vrk): This file has been running pretty wild and free, and it's likely
 // that a lot of the functions can be simplified and made more elegant. Revisit
 // after other audio cleanup is done. (crbug.com/120319)
 
 #include <algorithm>
 #include <limits>
 
 #include "base/atomicops.h"
 #include "base/basictypes.h"
 #include "base/logging.h"
 #include "base/shared_memory.h"
 #include "base/time.h"
 #if defined(OS_WIN)
+#include "base/sys_info.h"
 #include "base/win/windows_version.h"
 #include "media/audio/audio_manager_base.h"
 #endif
 #include "media/audio/audio_parameters.h"
 #include "media/audio/audio_util.h"
 #if defined(OS_MACOSX)
 #include "media/audio/mac/audio_low_latency_input_mac.h"
 #include "media/audio/mac/audio_low_latency_output_mac.h"
 #endif
 #if defined(OS_WIN)
@@ skipping 478 matching lines @@
 }
 
 #if defined(OS_WIN)
 
 bool IsWASAPISupported() {
   // Note: that function correctly returns that Windows Server 2003 does not
   // support WASAPI.
   return base::win::GetVersion() >= base::win::VERSION_VISTA;
 }
 
+int NumberOfWaveOutBuffers() {
+  // Simple heuristic: use 3 buffers on single-core system or on Vista,
+  // 2 otherwise.
+  // Entire Windows audio stack was rewritten for Windows Vista, and wave out
+  // API is simulated on top of new API, so there is noticeable performance
+  // degradation compared to Windows XP. Part of regression was fixed in
+  // Windows 7. Maybe it is fixed in Vista Serice Pack, but let's be cautious.
+  if ((base::SysInfo::NumberOfProcessors() < 2) ||
+      (base::win::GetVersion() == base::win::VERSION_VISTA)) {
+    return 3;
+  }
+  return 2;
+}
+
 #endif
 
 }  // namespace media