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

content/browser/browser_main_runner.cc

Index: content/browser/browser_main_runner.cc
diff --git a/content/browser/browser_main_runner.cc b/content/browser/browser_main_runner.cc
index 96a3454f1499edbb0211afbcdb1ba8024cb6e2bc..857824a4eeb0a95280e66bce5aa2815a2cd8719a 100644
--- a/content/browser/browser_main_runner.cc
+++ b/content/browser/browser_main_runner.cc
@@ -11,6 +11,7 @@
 #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"
@@ -60,6 +61,15 @@ class BrowserMainRunnerImpl : public BrowserMainRunner {
   }
 
   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");
 
@@ -224,6 +234,13 @@ class BrowserMainRunnerImpl : public BrowserMainRunner {
 
       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;
     }
   }