Reduce overhead of Chrome's ETW tracing to make it more usable.

base/trace_event/trace_event_etw_export_win.cc

 // Copyright 2015 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 "base/trace_event/trace_event_etw_export_win.h"
 
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "base/memory/singleton.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 114 matching lines @@
     int num_args,
     const char** arg_names,
     const unsigned char* arg_types,
     const unsigned long long* arg_values,
     const scoped_refptr<ConvertableToTraceFormat>* convertable_values) {
   // We bail early in case exporting is disabled or no consumer is listening.
   if (!GetInstance() || !GetInstance()->ETWExportEnabled_ ||
       !EventEnabledChromeEvent())
     return;
 
-  std::string phase_string;
+  const char* phase_string = nullptr;
+  // Space to store the phase identifier and null-terminator, when needed.
+  char phase_buffer[2];
   switch (phase) {
     case TRACE_EVENT_PHASE_BEGIN:
       phase_string = "Begin";
       break;
     case TRACE_EVENT_PHASE_END:
       phase_string = "End";
       break;
     case TRACE_EVENT_PHASE_COMPLETE:
       phase_string = "Complete";
       break;
@@ skipping 42 matching lines @@
     case TRACE_EVENT_PHASE_CREATE_OBJECT:
       phase_string = "Phase Create Object";
       break;
     case TRACE_EVENT_PHASE_SNAPSHOT_OBJECT:
       phase_string = "Phase Snapshot Object";
       break;
     case TRACE_EVENT_PHASE_DELETE_OBJECT:
       phase_string = "Phase Delete Object";
       break;
     default:
-      phase_string.push_back(phase);
+      phase_buffer[0] = phase;
+      phase_buffer[1] = 0;
+      phase_string = phase_buffer;
       break;
   }
 
   std::string arg_values_string[3];
   for (int i = 0; i < num_args; i++) {
     if (arg_types[i] == TRACE_VALUE_TYPE_CONVERTABLE) {
-      convertable_values[i]->AppendAsTraceFormat(arg_values_string + i);
+      // Temporarily do nothing here. This function consumes 1/3 to 1/2 of
+      // *total* process CPU time when ETW tracing, and many of the strings
+      // created exceed WPA's 4094 byte limit and are shown as:
+      // "Unable to parse data".

[Primiano Tucci (use gerrit), 2015/05/14 22:12:52]

Can you copy and paste the (excellent) commit message into a crbug and add a
reference  to crbug.com/XXXXX here (and in BUG= in the commit message)? (git
blame history tends to get hard to follow if the file gets moved)

[brucedawson, 2015/05/14 22:50:06]

Done.

+      //convertable_values[i]->AppendAsTraceFormat(arg_values_string + i);
     } else {
       TraceEvent::TraceValue trace_event;
       trace_event.as_uint = arg_values[i];
       TraceEvent::AppendValueAsJSON(arg_types[i], trace_event,
                                     arg_values_string + i);
     }
   }
 
   EventWriteChromeEvent(
-      name, phase_string.c_str(), num_args > 0 ? arg_names[0] : "",
+      name, phase_string, num_args > 0 ? arg_names[0] : "",
       arg_values_string[0].c_str(), num_args > 1 ? arg_names[1] : "",
       arg_values_string[1].c_str(), num_args > 2 ? arg_names[2] : "",
       arg_values_string[2].c_str());
 }
 
 // static
 void TraceEventETWExport::AddCustomEvent(const char* name,
                                          char const* phase,
                                          const char* arg_name_1,
                                          const char* arg_value_1,
                                          const char* arg_name_2,
                                          const char* arg_value_2,
                                          const char* arg_name_3,
                                          const char* arg_value_3) {
   if (!GetInstance() || !GetInstance()->ETWExportEnabled_ ||
       !EventEnabledChromeEvent())
     return;
 
   EventWriteChromeEvent(name, phase, arg_name_1, arg_value_1, arg_name_2,
                         arg_value_2, arg_name_3, arg_value_3);
 }
 
 void TraceEventETWExport::Resurrect() {
   StaticMemorySingletonTraits<TraceEventETWExport>::Resurrect();
 }
 
 }  // namespace trace_event
 }  // namespace base