Added support for 'spare' file that can be used at startup.

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/bind.h"
+#include "base/bind_helpers.h"
 #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/histogram_macros.h"
 #include "base/metrics/persistent_histogram_allocator.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "chrome/browser/metrics/chrome_metrics_service_client.h"
 #include "chrome/browser/metrics/chrome_metrics_services_manager_client.h"
 #include "chrome/browser/tracing/background_tracing_field_trial.h"
 #include "chrome/common/channel_info.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 {
 
+// Creating a "spare" file for persistent metrics involves a lot of I/O and
+// isn't important so delay the operation for a while after startup.
+#if defined(OS_ANDROID)
+// Android needs the spare file and also launches faster.
+constexpr bool kSpareFileRequired = true;
+constexpr int kSpareFileCreateDelaySeconds = 10;
+#else
+// Desktop may have to restore a lot of tabs so give it more time before doing
+// non-essential work.
+constexpr bool kSpareFileRequired = false;
+constexpr int kSpareFileCreateDelaySeconds = 90;

[Alexei Svitkine (slow), 2017/05/23 17:27:58]

If spare file is not required, why should we even create it on non-Android?

[bcwhite, 2017/05/24 17:01:50]

It's still a performance boost on all platforms, just not as significant.

[Alexei Svitkine (slow), 2017/05/24 17:09:53]

OK, please add this to the comment in the #else to provide this context.

[bcwhite, 2017/05/24 18:23:29]

Done.

+#endif
+
 // 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() {

[Alexei Svitkine (slow), 2017/05/23 17:27:58]

This function is getting more and more complicated. I'm OK with not doing it in
this CL, but let's move this code out of chrome_browser_field_trials.cc soon
(e.g. after this CL) - looks like there's already a TODO about it.

[bcwhite, 2017/05/24 17:01:50]

Sure.  I tried to split it into two methods but the data overlap was such that
pretty much everything had to be passed as a parameter or recalculated.

   base::FilePath metrics_dir;
   if (!base::PathService::Get(chrome::DIR_USER_DATA, &metrics_dir))
     return;
 
-  base::FilePath metrics_file, active_file;
+  base::FilePath metrics_file, active_file, spare_file;

[Alexei Svitkine (slow), 2017/05/23 17:27:58]

Nit: 1 per line.

[bcwhite, 2017/05/24 17:01:49]

Done.

   base::GlobalHistogramAllocator::ConstructFilePaths(
       metrics_dir, ChromeMetricsServiceClient::kBrowserMetricsName,
-      &metrics_file, &active_file);
+      &metrics_file, &active_file, &spare_file);
 
   // 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);
 
   // This is used to report results to an UMA histogram.
   enum InitResult {
     LOCAL_MEMORY_SUCCESS,
     LOCAL_MEMORY_FAILED,
     MAPPED_FILE_SUCCESS,
     MAPPED_FILE_FAILED,
     MAPPED_FILE_EXISTS,
+    NO_SPARE_FILE,
     INIT_RESULT_MAX
   };
   InitResult result;
 
   // 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, has the 99.9 percentile
   // around 4MiB as of 2017-02-16.
   const size_t kAllocSize = 8 << 20;     // 8 MiB
   const uint32_t kAllocId = 0x935DDD43;  // SHA1(BrowserMetrics)
   std::string storage = variations::GetVariationParamValueByFeature(
       base::kPersistentHistogramsFeature, "storage");
 
   if (storage.empty() || storage == "MappedFile") {
     // If for some reason the existing "active" file could not be moved above
     // then it is essential it be scheduled for deletion when possible and the
     // contents ignored. Because this shouldn't happen but can on an OS like
     // Windows where another process reading the file (backup, AV, etc.) can
     // prevent its alteration, it's necessary to handle this case by switching
     // to the equivalent of "LocalMemory" for this run.
     if (base::PathExists(active_file)) {
       base::File file(active_file, base::File::FLAG_OPEN |
                                        base::File::FLAG_READ |
                                        base::File::FLAG_DELETE_ON_CLOSE);
       result = MAPPED_FILE_EXISTS;
       base::GlobalHistogramAllocator::CreateWithLocalMemory(
           kAllocSize, kAllocId,
           ChromeMetricsServiceClient::kBrowserMetricsName);
     } else {
-      // Create global allocator with the "active" file.
-      if (base::GlobalHistogramAllocator::CreateWithFile(
-              active_file, kAllocSize, kAllocId,
-              ChromeMetricsServiceClient::kBrowserMetricsName)) {
+      // Move any sparse file into the active position.
+      base::ReplaceFile(spare_file, active_file, nullptr);
+      // Create global allocator using the "active" file.
+      if (kSpareFileRequired && !base::PathExists(active_file)) {
+        result = NO_SPARE_FILE;
+        base::GlobalHistogramAllocator::CreateWithLocalMemory(
+            kAllocSize, kAllocId,
+            ChromeMetricsServiceClient::kBrowserMetricsName);
+      } else if (base::GlobalHistogramAllocator::CreateWithFile(
+                     active_file, kAllocSize, kAllocId,
+                     ChromeMetricsServiceClient::kBrowserMetricsName)) {
         result = MAPPED_FILE_SUCCESS;
       } else {
         result = MAPPED_FILE_FAILED;
       }
-    }
+    }  // clang-format on

[Alexei Svitkine (slow), 2017/05/23 17:27:58]

What's this comment for?

[bcwhite, 2017/05/24 17:01:49]

Leftover from previous organization where clang-format couldn't get it right. 
Removed.

+    // Schedule the creation of a "spare" file for use on the next run.
+    base::PostDelayedTaskWithTraits(
+        FROM_HERE,
+        {base::MayBlock(), base::TaskPriority::LOWEST,
+         base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
+        base::BindOnce(base::IgnoreResult(
+                           &base::GlobalHistogramAllocator::CreateSpareFile),
+                       base::Passed(&spare_file), kAllocSize),
+        base::TimeDelta::FromSeconds(kSpareFileCreateDelaySeconds));
   } else if (storage == "LocalMemory") {
     // Use local memory for storage even though it will not persist across
     // an unclean shutdown.
     base::GlobalHistogramAllocator::CreateWithLocalMemory(
         kAllocSize, kAllocId, ChromeMetricsServiceClient::kBrowserMetricsName);
     result = LOCAL_MEMORY_SUCCESS;
   } else {
     // Persistent metric storage is disabled.
     return;
   }
@@ skipping 53 matching lines @@
     bool has_seed,
     base::FeatureList* feature_list) {
   CreateFallbackSamplingTrialIfNeeded(has_seed, feature_list);
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
   // Persistent histograms must be enabled as soon as possible.
   InstantiatePersistentHistograms();
   tracing::SetupBackgroundTracingFieldTrial();
 }