Add histograms to record the number of writes to the prefs file

base/prefs/json_pref_store.cc

Index: base/prefs/json_pref_store.cc
diff --git a/base/prefs/json_pref_store.cc b/base/prefs/json_pref_store.cc
index 47cd424d9cc9dfa7e8051d13ce72316a6bbd78b9..890ac138062b61590fbf11dd3a299557b4c9d3d5 100644
--- a/base/prefs/json_pref_store.cc
+++ b/base/prefs/json_pref_store.cc
@@ -16,6 +16,7 @@
 #include "base/metrics/histogram.h"
 #include "base/prefs/pref_filter.h"
 #include "base/sequenced_task_runner.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/task_runner_util.h"
 #include "base/threading/sequenced_worker_pool.h"
@@ -42,8 +43,42 @@ JsonPrefStore::ReadResult::ReadResult()
 JsonPrefStore::ReadResult::~ReadResult() {
 }
 
+class JsonPrefStore::WriteCountHistogram {
+ public:
+  WriteCountHistogram(const base::TimeDelta& commit_interval,
+                      const base::TimeDelta& report_interval,
+                      const base::FilePath& path);
+  ~WriteCountHistogram();
+
+  void RecordWriteOccured();
+
+ private:
+  base::HistogramBase* GetHistogram();
+
+  // The minimum interval at which writes can occur.
+  const base::TimeDelta commit_interval_;
+
+  // The interval at which to report write counts.
+  const base::TimeDelta report_interval_;
+
+  // The path to the file.
+  const base::FilePath path_;
+
+  // The time at which the last histogram value was reported for the number
+  // of write counts.
+  base::Time last_report_time_;
+
+  // The number of writes that have occured since the last write count was
+  // reported.
+  uint32_t writes_since_last_report_;
+
+  DISALLOW_COPY_AND_ASSIGN(WriteCountHistogram);
+};
+
 namespace {
 
+const int32 kHistogramWriteReportIntervalInMins = 5;
+
 // Some extensions we'll tack on to copies of the Preferences files.
 const base::FilePath::CharType kBadExtension[] = FILE_PATH_LITERAL("bad");
 
@@ -158,6 +193,11 @@ JsonPrefStore::JsonPrefStore(
       filtering_in_progress_(false),
       read_error_(PREF_READ_ERROR_NONE) {
   DCHECK(!path_.empty());
+
+  write_count_histogram_.reset(new WriteCountHistogram(

[Mattias Nissler (ping if slow), 2015/04/20 09:43:47]

looks like there's no good reason to keep this in a scoped_ptr now (as opposed
to just using a plain data member).

+      writer_.commit_interval(),
+      base::TimeDelta::FromMinutes(kHistogramWriteReportIntervalInMins),
+      path_));
 }
 
 JsonPrefStore::JsonPrefStore(
@@ -394,6 +434,8 @@ JsonPrefStore::~JsonPrefStore() {
 bool JsonPrefStore::SerializeData(std::string* output) {
   DCHECK(CalledOnValidThread());
 
+  write_count_histogram_->RecordWriteOccured();
+
   if (pref_filter_)
     pref_filter_->FilterSerializeData(prefs_.get());
 
@@ -435,3 +477,60 @@ void JsonPrefStore::FinalizeFileRead(bool initialization_successful,
 
   return;
 }
+
+JsonPrefStore::WriteCountHistogram::WriteCountHistogram(
+    const base::TimeDelta& commit_interval,
+    const base::TimeDelta& report_interval,
+    const base::FilePath& path)
+    : commit_interval_(commit_interval),
+      report_interval_(report_interval),
+      path_(path),
+      last_report_time_(base::Time::Now()),
+      writes_since_last_report_(0) {
+}
+
+JsonPrefStore::WriteCountHistogram::~WriteCountHistogram() {
+  // Record any outstanding writes before shutdown.
+  GetHistogram()->Add(writes_since_last_report_);
+}
+
+void JsonPrefStore::WriteCountHistogram::RecordWriteOccured() {
+  base::Time current_time = base::Time::Now();
+  base::TimeDelta time_since_last_report = current_time - last_report_time_;
+  base::HistogramBase* histogram = GetHistogram();
+
+  // If the time since the last report exceeds the report interval, we report
+  // all of the fully elapsed intervals up to the current time.
+  while (time_since_last_report > report_interval_) {
+    histogram->Add(writes_since_last_report_);
+
+    writes_since_last_report_ = 0;
+    last_report_time_ += report_interval_;
+    time_since_last_report = current_time - last_report_time_;
+  }
+
+  writes_since_last_report_++;
+}
+
+base::HistogramBase* JsonPrefStore::WriteCountHistogram::GetHistogram() {
+  std::string spaceless_basename;
+  base::ReplaceChars(path_.BaseName().MaybeAsASCII(), " ", "_",
+                     &spaceless_basename);
+  std::string histogram_name =
+      "Settings.JsonDataWriteCount." + spaceless_basename;
+
+  // The min value for a histogram is 1. The max value is the maximum number of
+  // writes that can occur in the window being recorded. The number of buckets
+  // used is the max value (plus the underflow/overflow buckets).
+  int32_t min_value = 1;
+  int32_t max_value = report_interval_ / commit_interval_;
+  int32_t num_buckets = max_value + 2;
+
+  // The histogram below is an expansion of the UMA_HISTOGRAM_CUSTOM_COUNTS
+  // macro adapted to allow for a dynamically suffixed histogram name.
+  // Note: The factory creates and owns the histogram.
+  base::HistogramBase* histogram = base::Histogram::FactoryGet(
+      histogram_name, min_value, max_value, num_buckets,
+      base::HistogramBase::kUmaTargetedHistogramFlag);
+  return histogram;
+}