Support saving browser metrics to disk and reading them during next run.

content/browser/browser_main_runner.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/public/browser/browser_main_runner.h"
 
 #include "base/base_switches.h"
 #include "base/command_line.h"
 #include "base/debug/leak_annotations.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/metrics/persistent_histogram_allocator.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/profiler/scoped_profile.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/time/time.h"
 #include "base/trace_event/heap_profiler_allocation_context_tracker.h"
 #include "base/trace_event/trace_event.h"
 #include "base/tracked_objects.h"
 #include "build/build_config.h"
 #include "components/tracing/trace_config_file.h"
 #include "components/tracing/tracing_switches.h"
@@ skipping 29 matching lines @@
  public:
   BrowserMainRunnerImpl()
       : initialization_started_(false), is_shutdown_(false) {}
 
   ~BrowserMainRunnerImpl() override {
     if (initialization_started_ && !is_shutdown_)
       Shutdown();
   }
 
   int Initialize(const MainFunctionParams& parameters) override {
+    // Create persistent/shared memory and allow histograms to be stored in it.
+    // Memory that is not actualy used won't be physically mapped by the system.
+    // BrowserMetrics usage peaked around 95% of 2MiB as of 2016-02-20.

[Ilya Sherman, 2016/04/25 19:48:41]

nit: "95% of 2MiB" is confusing -- it kind of sounds like it might mean 1.9MiB
(since 1.9 is 95% of 2).  Could you please rephrase this bit?

[bcwhite, 2016/04/25 20:37:17]

That is what it means.  Better to just say "1.9MiB".

+    base::GlobalHistogramAllocator::CreateWithLocalMemory(
+        3 << 20,     // 3 MiB
+        0x935DDD43,  // SHA1(BrowserMetrics)
+        "BrowserMetrics");
+    base::GlobalHistogramAllocator::Disable();  // Enabled by experiment only.
+
     SCOPED_UMA_HISTOGRAM_LONG_TIMER(
         "Startup.BrowserMainRunnerImplInitializeLongTime");
 
     // TODO(vadimt, yiyaoliu): Remove all tracked_objects references below once
     // crbug.com/453640 is fixed.
     tracked_objects::ThreadData::InitializeThreadContext(kMainThreadName);
     base::trace_event::AllocationContextTracker::SetCurrentThreadName(
         kMainThreadName);
     TRACK_SCOPED_REGION(
         "Startup", "BrowserMainRunnerImpl::Initialize");
@@ skipping 144 matching lines @@
       // Forcefully terminates the RunLoop inside MessagePumpForUI, ensuring
       // proper shutdown for content_browsertests. Shutdown() is not used by
       // the actual browser.
       if (base::MessageLoop::current()->is_running())
         base::MessageLoop::current()->QuitNow();
   #endif
       main_loop_.reset(NULL);
 
       notification_service_.reset(NULL);
 
+      // Dump all the persistent metrics to a file for upload during the next
+      // run of the browser.
+      base::GlobalHistogramAllocator* allocator =
+          base::GlobalHistogramAllocator::Get();
+      if (allocator)

[Ilya Sherman, 2016/04/25 19:48:41]

Under what circumstances can the allocator be null here?

[bcwhite, 2016/04/25 20:37:17]

It will be null if the allocator has been disabled.

[Ilya Sherman, 2016/04/25 21:12:11]

Yikes, that is so confusing that it confused me even immediately after having
read through this CL.  I'd much rather test allocator->IsEnabled() here... or
else internally to WriteToPersistentLocation().

[bcwhite, 2016/04/26 13:32:30]

Acknowledged.

+        allocator->WriteToPersistentLocation();
+
       is_shutdown_ = true;
     }
   }
 
  protected:
   // True if we have started to initialize the runner.
   bool initialization_started_;
 
   // True if the runner has been shut down.
   bool is_shutdown_;
@@ skipping 12 matching lines @@
 BrowserMainRunner* BrowserMainRunner::Create() {
   return new BrowserMainRunnerImpl();
 }
 
 // static
 bool BrowserMainRunner::ExitedMainMessageLoop() {
   return g_exited_main_message_loop;
 }
 
 }  // namespace content