Support experiment to store persistent metrics in memory-mapped file.

chrome/browser/chrome_browser_field_trials.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 "chrome/browser/chrome_browser_field_trials.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/feature_list.h"
+#include "base/files/file_util.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/persistent_histogram_allocator.h"
+#include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "chrome/browser/metrics/chrome_metrics_service_client.h"
 #include "chrome/browser/tracing/background_tracing_field_trial.h"
+#include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_switches.h"
 #include "components/metrics/metrics_pref_names.h"
+#include "components/variations/variations_associated_data.h"
 
 #if defined(OS_ANDROID)
 #include "chrome/browser/chrome_browser_field_trials_mobile.h"
 #else
 #include "chrome/browser/chrome_browser_field_trials_desktop.h"
 #endif
 
 namespace {
 
 // Check for feature enabling the use of persistent histogram storage and
 // enable the global allocator if so.
+// TODO(bcwhite): Move this and CreateInstallerFileMetricsProvider into a new
+// file and make kBrowserMetricsName local to that file.
 void InstantiatePersistentHistograms() {
-  if (base::FeatureList::IsEnabled(base::kPersistentHistogramsFeature)) {
-    // 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, as reported in UMA, peaked around 1.9MiB
-    // as of 2016-02-20.
+  base::FilePath metrics_dir;
+  if (!base::PathService::Get(chrome::DIR_USER_DATA, &metrics_dir))
+    return;
+
+  base::FilePath metrics_file =
+      metrics_dir
+          .AppendASCII(ChromeMetricsServiceClient::kBrowserMetricsName)
+          .AddExtension(base::PersistentMemoryAllocator::kFileExtension);
+  base::FilePath active_file =
+      metrics_dir
+          .AppendASCII(
+              std::string(ChromeMetricsServiceClient::kBrowserMetricsName) +
+              "-active")
+          .AddExtension(base::PersistentMemoryAllocator::kFileExtension);
+
+  // Move any existing "active" file to the final name from which it will be
+  // read when reporting initial stability metrics. If there is no file to
+  // move, remove any old, existing file from before the previous session.
+  if (!base::ReplaceFile(active_file, metrics_file, nullptr))
+    base::DeleteFile(metrics_file, /*recursive=*/false);
+
+  // 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, as reported in UMA, peaked around 1.9MiB
+  // as of 2016-02-20.
+  const size_t kAllocSize = 3 << 20;     // 3 MiB

[Ilya Sherman, 2016/06/29 20:41:38]

Unrelated to this CL: What happens if the alloc size is exceeded, by the way?

[bcwhite, 2016/06/30 22:56:12]

In that case, new histograms will get allocated from the heap.  Everything will
run as normal except those on the heap will not be persisted.

[Ilya Sherman, 2016/06/30 23:16:48]

Okay, sounds good.  Do we have metrics in place to measure how frequently this
occurs in practice?

[bcwhite, 2016/07/11 18:06:39]

Not directly.  There is a measure of how much of the memory segment is used. 
There is also a measure of how many histograms are allocated from each of heap
and persistent memory.

+  const uint32_t kAllocId = 0x935DDD43;  // SHA1(BrowserMetrics)
+  std::string storage = variations::GetVariationParamValueByFeature(
+      base::kPersistentHistogramsFeature, "storage");
+  if (storage == "MappedFile") {
+    // Create global allocator with the "active" file.
+    base::GlobalHistogramAllocator::CreateWithFile(
+        active_file, kAllocSize, kAllocId,
+        ChromeMetricsServiceClient::kBrowserMetricsName);
+  } else if (storage == "LocalMemory") {
+    // Use local memory for storage even though it will not persist across
+    // an unclean shutdown.
     base::GlobalHistogramAllocator::CreateWithLocalMemory(
-        3 << 20,     // 3 MiB
-        0x935DDD43,  // SHA1(BrowserMetrics)
-        ChromeMetricsServiceClient::kBrowserMetricsName);
-    base::GlobalHistogramAllocator* allocator =
-        base::GlobalHistogramAllocator::Get();
-    allocator->CreateTrackingHistograms(
-        ChromeMetricsServiceClient::kBrowserMetricsName);
+        kAllocSize, kAllocId, ChromeMetricsServiceClient::kBrowserMetricsName);
+  } else {
+    // Persistent metric storage is disabled.
+    return;
   }
+
+  base::GlobalHistogramAllocator* allocator =
+      base::GlobalHistogramAllocator::Get();
+  allocator->CreateTrackingHistograms(
+      ChromeMetricsServiceClient::kBrowserMetricsName);
+  allocator->SetPersistentLocation(active_file);
 }
 
 }  // namespace
 
 ChromeBrowserFieldTrials::ChromeBrowserFieldTrials(
     const base::CommandLine& parsed_command_line)
     : parsed_command_line_(parsed_command_line) {
 }
 
 ChromeBrowserFieldTrials::~ChromeBrowserFieldTrials() {
 }
 
 void ChromeBrowserFieldTrials::SetupFieldTrials() {
   // Field trials that are shared by all platforms.
   InstantiateDynamicTrials();
 
 #if defined(OS_ANDROID)
   chrome::SetupMobileFieldTrials(parsed_command_line_);
 #else
   chrome::SetupDesktopFieldTrials(parsed_command_line_);
 #endif
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
   // Persistent histograms must be enabled as soon as possible.
   InstantiatePersistentHistograms();
   tracing::SetupBackgroundTracingFieldTrial();
 }