Add support for persistent sparse histograms.

Sparse histograms don't have a single "values vector" that
can be created during construction.  Rather, they create a
list of value "records" that can be easily added to as needed.

BUG=546019
TBR=mark
mark: gn and gyp (new files)

Review URL: https://codereview.chromium.org/1734033003

Cr-Commit-Position: refs/heads/master@{#381699}

14 files changed, 964 insertions, 142 deletions

diff --git a/base/BUILD.gn b/base/BUILD.gn
index e1b1d9d..1cbc063 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -536,6 +536,8 @@ component("base") {
     "metrics/persistent_histogram_allocator.h",
     "metrics/persistent_memory_allocator.cc",
     "metrics/persistent_memory_allocator.h",
+    "metrics/persistent_sample_map.cc",
+    "metrics/persistent_sample_map.h",
     "metrics/sample_map.cc",
     "metrics/sample_map.h",
     "metrics/sample_vector.cc",
@@ -1756,6 +1758,7 @@ test("base_unittests") {
     "metrics/metrics_hashes_unittest.cc",
     "metrics/persistent_histogram_allocator_unittest.cc",
     "metrics/persistent_memory_allocator_unittest.cc",
+    "metrics/persistent_sample_map_unittest.cc",
     "metrics/sample_map_unittest.cc",
     "metrics/sample_vector_unittest.cc",
     "metrics/sparse_histogram_unittest.cc",
diff --git a/base/base.gyp b/base/base.gyp
index 88e27f1..d7f3519 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -492,6 +492,7 @@
         'metrics/metrics_hashes_unittest.cc',
         'metrics/persistent_histogram_allocator_unittest.cc',
         'metrics/persistent_memory_allocator_unittest.cc',
+        'metrics/persistent_sample_map_unittest.cc',
         'metrics/sample_map_unittest.cc',
         'metrics/sample_vector_unittest.cc',
         'metrics/sparse_histogram_unittest.cc',
diff --git a/base/base.gypi b/base/base.gypi
index 7ef9266..5d7693f 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -416,6 +416,8 @@
           'metrics/persistent_histogram_allocator.h',
           'metrics/persistent_memory_allocator.cc',
           'metrics/persistent_memory_allocator.h',
+          'metrics/persistent_sample_map.cc',
+          'metrics/persistent_sample_map.h',
           'metrics/sample_map.cc',
           'metrics/sample_map.h',
           'metrics/sample_vector.cc',
diff --git a/base/metrics/histogram.cc b/base/metrics/histogram.cc
index cd6d6be..35197e2 100644
--- a/base/metrics/histogram.cc
+++ b/base/metrics/histogram.cc
@@ -127,10 +127,9 @@ class Histogram::Factory {
   }
 
   // Perform any required datafill on the just-created histogram.  If
-  // overridden, be sure to call the "super" version.
-  virtual void FillHistogram(HistogramBase* histogram) {
-    histogram->SetFlags(flags_);
-  }
+  // overridden, be sure to call the "super" version -- this method may not
+  // always remain empty.
+  virtual void FillHistogram(HistogramBase* histogram) {}
 
   // These values are protected (instead of private) because they need to
   // be accessible to methods of sub-classes in order to avoid passing
@@ -172,7 +171,7 @@ HistogramBase* Histogram::Factory::Build() {
     }
 
     // Try to create the histogram using a "persistent" allocator. As of
-    // 2015-01-14, the availability of such is controlled by a base::Feature
+    // 2016-02-25, the availability of such is controlled by a base::Feature
     // that is off by default. If the allocator doesn't exist or if
     // allocating from it fails, code below will allocate the histogram from
     // the process heap.
@@ -181,7 +180,6 @@ HistogramBase* Histogram::Factory::Build() {
     PersistentHistogramAllocator* allocator =
         PersistentHistogramAllocator::GetGlobalAllocator();
     if (allocator) {
-      flags_ |= HistogramBase::kIsPersistent;
       tentative_histogram = allocator->AllocateHistogram(
           histogram_type_,
           name_,
@@ -199,6 +197,7 @@ HistogramBase* Histogram::Factory::Build() {
       DCHECK(!allocator);  // Shouldn't have failed.
       flags_ &= ~HistogramBase::kIsPersistent;
       tentative_histogram = HeapAlloc(registered_ranges);
+      tentative_histogram->SetFlags(flags_);
     }
 
     FillHistogram(tentative_histogram.get());
diff --git a/base/metrics/histogram_unittest.cc b/base/metrics/histogram_unittest.cc
index f621ec5..03dc7bd 100644
--- a/base/metrics/histogram_unittest.cc
+++ b/base/metrics/histogram_unittest.cc
@@ -37,11 +37,12 @@ class HistogramTest : public testing::TestWithParam<bool> {
   HistogramTest() : use_persistent_histogram_allocator_(GetParam()) {}
 
   void SetUp() override {
+    if (use_persistent_histogram_allocator_)
+      CreatePersistentHistogramAllocator();
+
     // Each test will have a clean state (no Histogram / BucketRanges
     // registered).
     InitializeStatisticsRecorder();
-    if (use_persistent_histogram_allocator_)
-      CreatePersistentHistogramAllocator();
   }
 
   void TearDown() override {
@@ -69,14 +70,8 @@ class HistogramTest : public testing::TestWithParam<bool> {
     // any persistent memory segment (which simplifies some tests).
     PersistentHistogramAllocator::GetCreateHistogramResultHistogram();
 
-    if (!allocator_memory_)
-      allocator_memory_.reset(new char[kAllocatorMemorySize]);
-
-    PersistentHistogramAllocator::ReleaseGlobalAllocatorForTesting();
-    memset(allocator_memory_.get(), 0, kAllocatorMemorySize);
-    PersistentHistogramAllocator::CreateGlobalAllocatorOnPersistentMemory(
-        allocator_memory_.get(), kAllocatorMemorySize, 0, 0,
-        "HistogramAllocatorTest");
+    PersistentHistogramAllocator::CreateGlobalAllocatorOnLocalMemory(
+        kAllocatorMemorySize, 0, "HistogramAllocatorTest");
     allocator_ =
         PersistentHistogramAllocator::GetGlobalAllocator()->memory_allocator();
   }
diff --git a/base/metrics/persistent_histogram_allocator.cc b/base/metrics/persistent_histogram_allocator.cc
index 2e4029e..6006d31 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -10,6 +10,7 @@
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
+#include "base/metrics/sparse_histogram.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/synchronization/lock.h"
 
@@ -71,12 +72,12 @@ scoped_ptr<BucketRanges> CreateRangesFromData(
 size_t CalculateRequiredCountsBytes(size_t bucket_count) {
   // 2 because each "sample count" also requires a backup "logged count"
   // used for calculating the delta during snapshot operations.
-  const unsigned kBytesPerBucket = 2 * sizeof(HistogramBase::AtomicCount);
+  const size_t kBytesPerBucket = 2 * sizeof(HistogramBase::AtomicCount);
 
   // If the |bucket_count| is such that it would overflow the return type,
   // perhaps as the result of a malicious actor, then return zero to
   // indicate the problem to the caller.
-  if (bucket_count > std::numeric_limits<uint32_t>::max() / kBytesPerBucket)
+  if (bucket_count > std::numeric_limits<size_t>::max() / kBytesPerBucket)
     return 0;
 
   return bucket_count * kBytesPerBucket;
@@ -276,6 +277,18 @@ scoped_ptr<HistogramBase> PersistentHistogramAllocator::CreateHistogram(
     return nullptr;
   }
 
+  // Sparse histograms are quite different so handle them as a special case.
+  if (histogram_data_ptr->histogram_type == SPARSE_HISTOGRAM) {
+    scoped_ptr<HistogramBase> histogram = SparseHistogram::PersistentCreate(
+        memory_allocator(), histogram_data_ptr->name,
+        &histogram_data_ptr->samples_metadata,
+        &histogram_data_ptr->logged_metadata);
+    DCHECK(histogram);
+    histogram->SetFlags(histogram_data_ptr->flags);
+    RecordCreateHistogramResult(CREATE_HISTOGRAM_SUCCESS);
+    return histogram;
+  }
+
   // Copy the histogram_data to local storage because anything in persistent
   // memory cannot be trusted as it could be changed at any moment by a
   // malicious actor that shares access. The contents of histogram_data are
@@ -318,7 +331,7 @@ scoped_ptr<HistogramBase> PersistentHistogramAllocator::CreateHistogram(
           histogram_data.counts_ref, kTypeIdCountsArray);
   size_t counts_bytes =
       CalculateRequiredCountsBytes(histogram_data.bucket_count);
-  if (!counts_data || !counts_bytes ||
+  if (!counts_data || counts_bytes == 0 ||
       memory_allocator_->GetAllocSize(histogram_data.counts_ref) <
           counts_bytes) {
     RecordCreateHistogramResult(CREATE_HISTOGRAM_INVALID_COUNTS_ARRAY);
@@ -445,49 +458,64 @@ scoped_ptr<HistogramBase> PersistentHistogramAllocator::AllocateHistogram(
     return nullptr;
   }
 
-  // If CalculateRequiredCountsBytes() returns zero then the bucket_count
-  // was not valid.
-  size_t bucket_count = bucket_ranges->bucket_count();
-  size_t counts_bytes = CalculateRequiredCountsBytes(bucket_count);
-  if (!counts_bytes) {
-    NOTREACHED();
-    return nullptr;
-  }
-
-  size_t ranges_bytes = (bucket_count + 1) * sizeof(HistogramBase::Sample);
-  PersistentMemoryAllocator::Reference ranges_ref =
-      memory_allocator_->Allocate(ranges_bytes, kTypeIdRangesArray);
-  PersistentMemoryAllocator::Reference counts_ref =
-      memory_allocator_->Allocate(counts_bytes, kTypeIdCountsArray);
+  // Create the metadata necessary for a persistent sparse histogram. This
+  // is done first because it is a small subset of what is required for
+  // other histograms.
   PersistentMemoryAllocator::Reference histogram_ref =
       memory_allocator_->Allocate(
           offsetof(PersistentHistogramData, name) + name.length() + 1,
           kTypeIdHistogram);
-  HistogramBase::Sample* ranges_data =
-      memory_allocator_->GetAsObject<HistogramBase::Sample>(ranges_ref,
-                                                            kTypeIdRangesArray);
   PersistentHistogramData* histogram_data =
       memory_allocator_->GetAsObject<PersistentHistogramData>(histogram_ref,
                                                               kTypeIdHistogram);
+  if (histogram_data) {
+    memcpy(histogram_data->name, name.c_str(), name.size() + 1);
+    histogram_data->histogram_type = histogram_type;
+    histogram_data->flags = flags | HistogramBase::kIsPersistent;
+  }
 
-  // Only continue here if all allocations were successful. If they weren't,
-  // there is no way to free the space but that's not really a problem since
-  // the allocations only fail because the space is full or corrupt and so
-  // any future attempts will also fail.
-  if (counts_ref && ranges_data && histogram_data) {
-    strcpy(histogram_data->name, name.c_str());
-    for (size_t i = 0; i < bucket_ranges->size(); ++i)
-      ranges_data[i] = bucket_ranges->range(i);
+  // Create the remaining metadata necessary for regular histograms.
+  if (histogram_type != SPARSE_HISTOGRAM) {
+    size_t bucket_count = bucket_ranges->bucket_count();
+    size_t counts_bytes = CalculateRequiredCountsBytes(bucket_count);
+    if (counts_bytes == 0) {
+      // |bucket_count| was out-of-range.
+      NOTREACHED();
+      return nullptr;
+    }
 
-    histogram_data->histogram_type = histogram_type;
-    histogram_data->flags = flags;
-    histogram_data->minimum = minimum;
-    histogram_data->maximum = maximum;
-    histogram_data->bucket_count = static_cast<uint32_t>(bucket_count);
-    histogram_data->ranges_ref = ranges_ref;
-    histogram_data->ranges_checksum = bucket_ranges->checksum();
-    histogram_data->counts_ref = counts_ref;
+    size_t ranges_bytes = (bucket_count + 1) * sizeof(HistogramBase::Sample);
+    PersistentMemoryAllocator::Reference counts_ref =
+        memory_allocator_->Allocate(counts_bytes, kTypeIdCountsArray);
+    PersistentMemoryAllocator::Reference ranges_ref =
+        memory_allocator_->Allocate(ranges_bytes, kTypeIdRangesArray);
+    HistogramBase::Sample* ranges_data =
+        memory_allocator_->GetAsObject<HistogramBase::Sample>(
+            ranges_ref, kTypeIdRangesArray);
+
+    // Only continue here if all allocations were successful. If they weren't,
+    // there is no way to free the space but that's not really a problem since
+    // the allocations only fail because the space is full or corrupt and so
+    // any future attempts will also fail.
+    if (counts_ref && ranges_data && histogram_data) {
+      for (size_t i = 0; i < bucket_ranges->size(); ++i)
+        ranges_data[i] = bucket_ranges->range(i);
+
+      histogram_data->minimum = minimum;
+      histogram_data->maximum = maximum;
+      // |bucket_count| must fit within 32-bits or the allocation of the counts
+      // array would have failed for being too large; the allocator supports
+      // less than 4GB total size.
+      histogram_data->bucket_count = static_cast<uint32_t>(bucket_count);
+      histogram_data->ranges_ref = ranges_ref;
+      histogram_data->ranges_checksum = bucket_ranges->checksum();
+      histogram_data->counts_ref = counts_ref;
+    } else {
+      histogram_data = nullptr;  // Clear this for proper handling below.
+    }
+  }
 
+  if (histogram_data) {
     // Create the histogram using resources in persistent memory. This ends up
     // resolving the "ref" values stored in histogram_data instad of just
     // using what is already known above but avoids duplicating the switch
diff --git a/base/metrics/persistent_sample_map.cc b/base/metrics/persistent_sample_map.cc
new file mode 100644
index 0000000..014a865
--- /dev/null
+++ b/base/metrics/persistent_sample_map.cc
@@ -0,0 +1,267 @@
+// Copyright 2016 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 "base/metrics/persistent_sample_map.h"
+
+#include "base/logging.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+typedef HistogramBase::Count Count;
+typedef HistogramBase::Sample Sample;
+
+namespace {
+
+// An iterator for going through a PersistentSampleMap. The logic here is
+// identical to that of SampleMapIterator but with different data structures.
+// Changes here likely need to be duplicated there.
+class PersistentSampleMapIterator : public SampleCountIterator {
+ public:
+  typedef std::map<HistogramBase::Sample, HistogramBase::Count*>
+      SampleToCountMap;
+
+  explicit PersistentSampleMapIterator(const SampleToCountMap& sample_counts);
+  ~PersistentSampleMapIterator() override;
+
+  // SampleCountIterator:
+  bool Done() const override;
+  void Next() override;
+  void Get(HistogramBase::Sample* min,
+           HistogramBase::Sample* max,
+           HistogramBase::Count* count) const override;
+
+ private:
+  void SkipEmptyBuckets();
+
+  SampleToCountMap::const_iterator iter_;
+  const SampleToCountMap::const_iterator end_;
+};
+
+PersistentSampleMapIterator::PersistentSampleMapIterator(
+    const SampleToCountMap& sample_counts)
+    : iter_(sample_counts.begin()),
+      end_(sample_counts.end()) {
+  SkipEmptyBuckets();
+}
+
+PersistentSampleMapIterator::~PersistentSampleMapIterator() {}
+
+bool PersistentSampleMapIterator::Done() const {
+  return iter_ == end_;
+}
+
+void PersistentSampleMapIterator::Next() {
+  DCHECK(!Done());
+  ++iter_;
+  SkipEmptyBuckets();
+}
+
+void PersistentSampleMapIterator::Get(Sample* min,
+                                      Sample* max,
+                                      Count* count) const {
+  DCHECK(!Done());
+  if (min)
+    *min = iter_->first;
+  if (max)
+    *max = iter_->first + 1;
+  if (count)
+    *count = *iter_->second;
+}
+
+void PersistentSampleMapIterator::SkipEmptyBuckets() {
+  while (!Done() && *iter_->second == 0) {
+    ++iter_;
+  }
+}
+
+// This structure holds an entry for a PersistentSampleMap within a persistent
+// memory allocator. The "id" must be unique across all maps held by an
+// allocator or they will get attached to the wrong sample map.
+struct SampleRecord {
+  uint64_t id;   // Unique identifier of owner.
+  Sample value;  // The value for which this record holds a count.
+  Count count;   // The count associated with the above value.
+};
+
+// The type-id used to identify sample records inside an allocator.
+const uint32_t kTypeIdSampleRecord = 0x8FE6A69F + 1;  // SHA1(SampleRecord) v1
+
+}  // namespace
+
+PersistentSampleMap::PersistentSampleMap(
+    uint64_t id,
+    PersistentMemoryAllocator* allocator,
+    Metadata* meta)
+    : HistogramSamples(id, meta),
+      allocator_(allocator) {
+  // This is created once but will continue to return new iterables even when
+  // it has previously reached the end.
+  allocator->CreateIterator(&sample_iter_);
+
+  // Load all existing samples during construction. It's no worse to do it
+  // here than at some point in the future and could be better if construction
+  // takes place on some background thread. New samples could be created at
+  // any time by parallel threads; if so, they'll get loaded when needed.
+  ImportSamples(kAllSamples);
+}
+
+PersistentSampleMap::~PersistentSampleMap() {}
+
+void PersistentSampleMap::Accumulate(Sample value, Count count) {
+  *GetOrCreateSampleCountStorage(value) += count;
+  IncreaseSum(static_cast<int64_t>(count) * value);
+  IncreaseRedundantCount(count);
+}
+
+Count PersistentSampleMap::GetCount(Sample value) const {
+  // Have to override "const" to make sure all samples have been loaded before
+  // being able to know what value to return.
+  Count* count_pointer =
+      const_cast<PersistentSampleMap*>(this)->GetSampleCountStorage(value);
+  return count_pointer ? *count_pointer : 0;
+}
+
+Count PersistentSampleMap::TotalCount() const {
+  // Have to override "const" in order to make sure all samples have been
+  // loaded before trying to iterate over the map.
+  const_cast<PersistentSampleMap*>(this)->ImportSamples(kAllSamples);
+
+  Count count = 0;
+  for (const auto& entry : sample_counts_) {
+    count += *entry.second;
+  }
+  return count;
+}
+
+scoped_ptr<SampleCountIterator> PersistentSampleMap::Iterator() const {
+  // Have to override "const" in order to make sure all samples have been
+  // loaded before trying to iterate over the map.
+  const_cast<PersistentSampleMap*>(this)->ImportSamples(kAllSamples);
+  return make_scoped_ptr(new PersistentSampleMapIterator(sample_counts_));
+}
+
+bool PersistentSampleMap::AddSubtractImpl(SampleCountIterator* iter,
+                                          Operator op) {
+  Sample min;
+  Sample max;
+  Count count;
+  for (; !iter->Done(); iter->Next()) {
+    iter->Get(&min, &max, &count);
+    if (min + 1 != max)
+      return false;  // SparseHistogram only supports bucket with size 1.
+
+    *GetOrCreateSampleCountStorage(min) +=
+        (op == HistogramSamples::ADD) ? count : -count;
+  }
+  return true;
+}
+
+Count* PersistentSampleMap::GetSampleCountStorage(Sample value) {
+  DCHECK_LE(0, value);
+
+  // If |value| is already in the map, just return that.
+  auto it = sample_counts_.find(value);
+  if (it != sample_counts_.end())
+    return it->second;
+
+  // Import any new samples from persistent memory looking for the value.
+  return ImportSamples(value);
+}
+
+Count* PersistentSampleMap::GetOrCreateSampleCountStorage(Sample value) {
+  // Get any existing count storage.
+  Count* count_pointer = GetSampleCountStorage(value);
+  if (count_pointer)
+    return count_pointer;
+
+  // Create a new record in persistent memory for the value.
+  PersistentMemoryAllocator::Reference ref =
+      allocator_->Allocate(sizeof(SampleRecord), kTypeIdSampleRecord);
+  SampleRecord* record =
+      allocator_->GetAsObject<SampleRecord>(ref, kTypeIdSampleRecord);
+  if (!record) {
+    // If the allocator was unable to create a record then it is full or
+    // corrupt. Instead, allocate the counter from the heap. This sample will
+    // not be persistent, will not be shared, and will leak but it's better
+    // than crashing.
+    NOTREACHED() << "full=" << allocator_->IsFull()
+                 << ", corrupt=" << allocator_->IsCorrupt();
+    count_pointer = new Count(0);
+    sample_counts_[value] = count_pointer;
+    return count_pointer;
+  }
+  record->id = id();
+  record->value = value;
+  record->count = 0;  // Should already be zero but don't trust other processes.
+  allocator_->MakeIterable(ref);
+
+  // A race condition between two independent processes (i.e. two independent
+  // histogram objects sharing the same sample data) could cause two of the
+  // above records to be created. The allocator, however, forces a strict
+  // ordering on iterable objects so use the import method to actually add the
+  // just-created record. This ensures that all PersistentSampleMap objects
+  // will always use the same record, whichever was first made iterable.
+  // Thread-safety within a process where multiple threads use the same
+  // histogram object is delegated to the controlling histogram object which,
+  // for sparse histograms, is a lock object.
+  count_pointer = ImportSamples(value);
+  DCHECK(count_pointer);
+  return count_pointer;
+}
+
+Count* PersistentSampleMap::ImportSamples(Sample until_value) {
+  // TODO(bcwhite): This import operates in O(V+N) total time per sparse
+  // histogram where V is the number of values for this object and N is
+  // the number of other iterable objects in the allocator. This becomes
+  // O(S*(SV+N)) or O(S^2*V + SN) overall where S is the number of sparse
+  // histograms.
+  //
+  // This is actually okay when histograms are expected to exist for the
+  // lifetime of the program, spreading the cost out, and S and V are
+  // relatively small, as is the current case.
+  //
+  // However, it is not so good for objects that are created, detroyed, and
+  // recreated on a periodic basis, such as when making a snapshot of
+  // sparse histograms owned by another, ongoing process. In that case, the
+  // entire cost is compressed into a single sequential operation... on the
+  // UI thread no less.
+  //
+  // This will be addressed in a future CL.
+
+  uint32_t type_id;
+  PersistentMemoryAllocator::Reference ref;
+  while ((ref = allocator_->GetNextIterable(&sample_iter_, &type_id)) != 0) {
+    if (type_id == kTypeIdSampleRecord) {
+      SampleRecord* record =
+          allocator_->GetAsObject<SampleRecord>(ref, kTypeIdSampleRecord);
+      if (!record)
+        continue;
+
+      // A sample record has been found but may not be for this histogram.
+      if (record->id != id())
+        continue;
+
+      // Check if the record's value is already known.
+      if (!ContainsKey(sample_counts_, record->value)) {
+        // No: Add it to map of known values if the value is valid.
+        if (record->value >= 0)
+          sample_counts_[record->value] = &record->count;
+      } else {
+        // Yes: Ignore it; it's a duplicate caused by a race condition -- see
+        // code & comment in GetOrCreateSampleCountStorage() for details.
+        // Check that nothing ever operated on the duplicate record.
+        DCHECK_EQ(0, record->count);
+      }
+
+      // Stop if it's the value being searched for.
+      if (record->value == until_value)
+        return &record->count;
+    }
+  }
+
+  return nullptr;
+}
+
+}  // namespace base
diff --git a/base/metrics/persistent_sample_map.h b/base/metrics/persistent_sample_map.h
new file mode 100644
index 0000000..a23b751
--- /dev/null
+++ b/base/metrics/persistent_sample_map.h
@@ -0,0 +1,78 @@
+// Copyright 2016 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.
+
+// PersistentSampleMap implements HistogramSamples interface. It is used
+// by the SparseHistogram class to store samples in persistent memory which
+// allows it to be shared between processes or live across restarts.
+
+#ifndef BASE_METRICS_PERSISTENT_SAMPLE_MAP_H_
+#define BASE_METRICS_PERSISTENT_SAMPLE_MAP_H_
+
+#include <stdint.h>
+
+#include <map>
+
+#include "base/compiler_specific.h"
+#include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_base.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/metrics/persistent_memory_allocator.h"
+
+namespace base {
+
+// The logic here is similar to that of SampleMap but with different data
+// structures. Changes here likely need to be duplicated there.
+class BASE_EXPORT PersistentSampleMap : public HistogramSamples {
+ public:
+  PersistentSampleMap(uint64_t id,
+                      PersistentMemoryAllocator* allocator,
+                      Metadata* meta);
+  ~PersistentSampleMap() override;
+
+  // HistogramSamples:
+  void Accumulate(HistogramBase::Sample value,
+                  HistogramBase::Count count) override;
+  HistogramBase::Count GetCount(HistogramBase::Sample value) const override;
+  HistogramBase::Count TotalCount() const override;
+  scoped_ptr<SampleCountIterator> Iterator() const override;
+
+ protected:
+  // Performs arithemetic. |op| is ADD or SUBTRACT.
+  bool AddSubtractImpl(SampleCountIterator* iter, Operator op) override;
+
+  // Gets a pointer to a "count" corresponding to a given |value|. Returns NULL
+  // if sample does not exist.
+  HistogramBase::Count* GetSampleCountStorage(HistogramBase::Sample value);
+
+  // Gets a pointer to a "count" corresponding to a given |value|, creating
+  // the sample (initialized to zero) if it does not already exists.
+  HistogramBase::Count* GetOrCreateSampleCountStorage(
+      HistogramBase::Sample value);
+
+ private:
+  enum : HistogramBase::Sample { kAllSamples = -1 };
+
+  // Imports samples from persistent memory by iterating over all sample
+  // records found therein, adding them to the sample_counts_ map. If a
+  // count for the sample |until_value| is found, stop the import and return
+  // a pointer to that counter. If that value is not found, null will be
+  // returned after all currently available samples have been loaded. Pass
+  // kAllSamples to force the importing of all available samples.
+  HistogramBase::Count* ImportSamples(HistogramBase::Sample until_value);
+
+  // All created/loaded sample values and their associated counts. The storage
+  // for the actual Count numbers is owned by the |allocator_|.
+  std::map<HistogramBase::Sample, HistogramBase::Count*> sample_counts_;
+
+  // The persistent memory allocator holding samples and an iterator through it.
+  PersistentMemoryAllocator* allocator_;
+  PersistentMemoryAllocator::Iterator sample_iter_;
+
+  DISALLOW_COPY_AND_ASSIGN(PersistentSampleMap);
+};
+
+}  // namespace base
+
+#endif  // BASE_METRICS_PERSISTENT_SAMPLE_MAP_H_
diff --git a/base/metrics/persistent_sample_map_unittest.cc b/base/metrics/persistent_sample_map_unittest.cc
new file mode 100644
index 0000000..c735f8f
--- /dev/null
+++ b/base/metrics/persistent_sample_map_unittest.cc
@@ -0,0 +1,243 @@
+// Copyright (c) 2016 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 "base/metrics/persistent_sample_map.h"
+
+#include "base/memory/scoped_ptr.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+namespace {
+
+TEST(PersistentSampleMapTest, AccumulateTest) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples(1, &allocator, meta);
+
+  samples.Accumulate(1, 100);
+  samples.Accumulate(2, 200);
+  samples.Accumulate(1, -200);
+  EXPECT_EQ(-100, samples.GetCount(1));
+  EXPECT_EQ(200, samples.GetCount(2));
+
+  EXPECT_EQ(300, samples.sum());
+  EXPECT_EQ(100, samples.TotalCount());
+  EXPECT_EQ(samples.redundant_count(), samples.TotalCount());
+}
+
+TEST(PersistentSampleMapTest, Accumulate_LargeValuesDontOverflow) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples(1, &allocator, meta);
+
+  samples.Accumulate(250000000, 100);
+  samples.Accumulate(500000000, 200);
+  samples.Accumulate(250000000, -200);
+  EXPECT_EQ(-100, samples.GetCount(250000000));
+  EXPECT_EQ(200, samples.GetCount(500000000));
+
+  EXPECT_EQ(75000000000LL, samples.sum());
+  EXPECT_EQ(100, samples.TotalCount());
+  EXPECT_EQ(samples.redundant_count(), samples.TotalCount());
+}
+
+TEST(PersistentSampleMapTest, AddSubtractTest) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta1 =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  HistogramSamples::Metadata* meta2 =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples1(1, &allocator, meta1);
+  PersistentSampleMap samples2(2, &allocator, meta2);
+
+  samples1.Accumulate(1, 100);
+  samples1.Accumulate(2, 100);
+  samples1.Accumulate(3, 100);
+
+  samples2.Accumulate(1, 200);
+  samples2.Accumulate(2, 200);
+  samples2.Accumulate(4, 200);
+
+  samples1.Add(samples2);
+  EXPECT_EQ(300, samples1.GetCount(1));
+  EXPECT_EQ(300, samples1.GetCount(2));
+  EXPECT_EQ(100, samples1.GetCount(3));
+  EXPECT_EQ(200, samples1.GetCount(4));
+  EXPECT_EQ(2000, samples1.sum());
+  EXPECT_EQ(900, samples1.TotalCount());
+  EXPECT_EQ(samples1.redundant_count(), samples1.TotalCount());
+
+  samples1.Subtract(samples2);
+  EXPECT_EQ(100, samples1.GetCount(1));
+  EXPECT_EQ(100, samples1.GetCount(2));
+  EXPECT_EQ(100, samples1.GetCount(3));
+  EXPECT_EQ(0, samples1.GetCount(4));
+  EXPECT_EQ(600, samples1.sum());
+  EXPECT_EQ(300, samples1.TotalCount());
+  EXPECT_EQ(samples1.redundant_count(), samples1.TotalCount());
+}
+
+TEST(PersistentSampleMapTest, PersistenceTest) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta12 =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples1(12, &allocator, meta12);
+  samples1.Accumulate(1, 100);
+  samples1.Accumulate(2, 200);
+  samples1.Accumulate(1, -200);
+  EXPECT_EQ(-100, samples1.GetCount(1));
+  EXPECT_EQ(200, samples1.GetCount(2));
+  EXPECT_EQ(300, samples1.sum());
+  EXPECT_EQ(100, samples1.TotalCount());
+  EXPECT_EQ(samples1.redundant_count(), samples1.TotalCount());
+
+  PersistentSampleMap samples2(12, &allocator, meta12);
+  EXPECT_EQ(samples1.id(), samples2.id());
+  EXPECT_EQ(samples1.sum(), samples2.sum());
+  EXPECT_EQ(samples1.redundant_count(), samples2.redundant_count());
+  EXPECT_EQ(samples1.TotalCount(), samples2.TotalCount());
+  EXPECT_EQ(-100, samples2.GetCount(1));
+  EXPECT_EQ(200, samples2.GetCount(2));
+  EXPECT_EQ(300, samples2.sum());
+  EXPECT_EQ(100, samples2.TotalCount());
+  EXPECT_EQ(samples2.redundant_count(), samples2.TotalCount());
+
+  EXPECT_EQ(0, samples2.GetCount(3));
+  EXPECT_EQ(0, samples1.GetCount(3));
+  samples2.Accumulate(3, 300);
+  EXPECT_EQ(300, samples2.GetCount(3));
+  EXPECT_EQ(300, samples1.GetCount(3));
+  EXPECT_EQ(samples1.sum(), samples2.sum());
+  EXPECT_EQ(samples1.redundant_count(), samples2.redundant_count());
+  EXPECT_EQ(samples1.TotalCount(), samples2.TotalCount());
+}
+
+TEST(PersistentSampleMapIteratorTest, IterateTest) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples(1, &allocator, meta);
+  samples.Accumulate(1, 100);
+  samples.Accumulate(2, 200);
+  samples.Accumulate(4, -300);
+  samples.Accumulate(5, 0);
+
+  scoped_ptr<SampleCountIterator> it = samples.Iterator();
+
+  HistogramBase::Sample min;
+  HistogramBase::Sample max;
+  HistogramBase::Count count;
+
+  it->Get(&min, &max, &count);
+  EXPECT_EQ(1, min);
+  EXPECT_EQ(2, max);
+  EXPECT_EQ(100, count);
+  EXPECT_FALSE(it->GetBucketIndex(NULL));
+
+  it->Next();
+  it->Get(&min, &max, &count);
+  EXPECT_EQ(2, min);
+  EXPECT_EQ(3, max);
+  EXPECT_EQ(200, count);
+
+  it->Next();
+  it->Get(&min, &max, &count);
+  EXPECT_EQ(4, min);
+  EXPECT_EQ(5, max);
+  EXPECT_EQ(-300, count);
+
+  it->Next();
+  EXPECT_TRUE(it->Done());
+}
+
+TEST(PersistentSampleMapIteratorTest, SkipEmptyRanges) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples(1, &allocator, meta);
+  samples.Accumulate(5, 1);
+  samples.Accumulate(10, 2);
+  samples.Accumulate(15, 3);
+  samples.Accumulate(20, 4);
+  samples.Accumulate(25, 5);
+
+  HistogramSamples::Metadata* meta2 =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples2(2, &allocator, meta2);
+  samples2.Accumulate(5, 1);
+  samples2.Accumulate(20, 4);
+  samples2.Accumulate(25, 5);
+
+  samples.Subtract(samples2);
+
+  scoped_ptr<SampleCountIterator> it = samples.Iterator();
+  EXPECT_FALSE(it->Done());
+
+  HistogramBase::Sample min;
+  HistogramBase::Sample max;
+  HistogramBase::Count count;
+
+  it->Get(&min, &max, &count);
+  EXPECT_EQ(10, min);
+  EXPECT_EQ(11, max);
+  EXPECT_EQ(2, count);
+
+  it->Next();
+  EXPECT_FALSE(it->Done());
+
+  it->Get(&min, &max, &count);
+  EXPECT_EQ(15, min);
+  EXPECT_EQ(16, max);
+  EXPECT_EQ(3, count);
+
+  it->Next();
+  EXPECT_TRUE(it->Done());
+}
+
+// Only run this test on builds that support catching a DCHECK crash.
+#if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)) && GTEST_HAS_DEATH_TEST
+TEST(PersistentSampleMapIteratorDeathTest, IterateDoneTest) {
+  LocalPersistentMemoryAllocator allocator(64 << 10, 0, "");  // 64 KiB
+
+  HistogramSamples::Metadata* meta =
+      allocator.GetAsObject<HistogramSamples::Metadata>(
+          allocator.Allocate(sizeof(HistogramSamples::Metadata), 0), 0);
+  PersistentSampleMap samples(1, &allocator, meta);
+
+  scoped_ptr<SampleCountIterator> it = samples.Iterator();
+
+  EXPECT_TRUE(it->Done());
+
+  HistogramBase::Sample min;
+  HistogramBase::Sample max;
+  HistogramBase::Count count;
+  EXPECT_DEATH(it->Get(&min, &max, &count), "");
+
+  EXPECT_DEATH(it->Next(), "");
+
+  samples.Accumulate(1, 100);
+  it = samples.Iterator();
+  EXPECT_FALSE(it->Done());
+}
+#endif
+// (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)) && GTEST_HAS_DEATH_TEST
+
+}  // namespace
+}  // namespace base
diff --git a/base/metrics/sample_map.cc b/base/metrics/sample_map.cc
index e276b91..21a4e35 100644
--- a/base/metrics/sample_map.cc
+++ b/base/metrics/sample_map.cc
@@ -5,12 +5,76 @@
 #include "base/metrics/sample_map.h"
 
 #include "base/logging.h"
+#include "base/stl_util.h"
 
 namespace base {
 
 typedef HistogramBase::Count Count;
 typedef HistogramBase::Sample Sample;
 
+namespace {
+
+// An iterator for going through a SampleMap. The logic here is identical
+// to that of PersistentSampleMapIterator but with different data structures.
+// Changes here likely need to be duplicated there.
+class SampleMapIterator : public SampleCountIterator {
+ public:
+  typedef std::map<HistogramBase::Sample, HistogramBase::Count>
+      SampleToCountMap;
+
+  explicit SampleMapIterator(const SampleToCountMap& sample_counts);
+  ~SampleMapIterator() override;
+
+  // SampleCountIterator:
+  bool Done() const override;
+  void Next() override;
+  void Get(HistogramBase::Sample* min,
+           HistogramBase::Sample* max,
+           HistogramBase::Count* count) const override;
+
+ private:
+  void SkipEmptyBuckets();
+
+  SampleToCountMap::const_iterator iter_;
+  const SampleToCountMap::const_iterator end_;
+};
+
+SampleMapIterator::SampleMapIterator(const SampleToCountMap& sample_counts)
+    : iter_(sample_counts.begin()),
+      end_(sample_counts.end()) {
+  SkipEmptyBuckets();
+}
+
+SampleMapIterator::~SampleMapIterator() {}
+
+bool SampleMapIterator::Done() const {
+  return iter_ == end_;
+}
+
+void SampleMapIterator::Next() {
+  DCHECK(!Done());
+  ++iter_;
+  SkipEmptyBuckets();
+}
+
+void SampleMapIterator::Get(Sample* min, Sample* max, Count* count) const {
+  DCHECK(!Done());
+  if (min)
+    *min = iter_->first;
+  if (max)
+    *max = iter_->first + 1;
+  if (count)
+    *count = iter_->second;
+}
+
+void SampleMapIterator::SkipEmptyBuckets() {
+  while (!Done() && iter_->second == 0) {
+    ++iter_;
+  }
+}
+
+}  // namespace
+
 SampleMap::SampleMap() : SampleMap(0) {}
 
 SampleMap::SampleMap(uint64_t id) : HistogramSamples(id) {}
@@ -39,11 +103,10 @@ Count SampleMap::TotalCount() const {
 }
 
 scoped_ptr<SampleCountIterator> SampleMap::Iterator() const {
-  return scoped_ptr<SampleCountIterator>(new SampleMapIterator(sample_counts_));
+  return make_scoped_ptr(new SampleMapIterator(sample_counts_));
 }
 
-bool SampleMap::AddSubtractImpl(SampleCountIterator* iter,
-                                HistogramSamples::Operator op) {
+bool SampleMap::AddSubtractImpl(SampleCountIterator* iter, Operator op) {
   Sample min;
   Sample max;
   Count count;
@@ -57,38 +120,4 @@ bool SampleMap::AddSubtractImpl(SampleCountIterator* iter,
   return true;
 }
 
-SampleMapIterator::SampleMapIterator(const SampleToCountMap& sample_counts)
-    : iter_(sample_counts.begin()),
-      end_(sample_counts.end()) {
-  SkipEmptyBuckets();
-}
-
-SampleMapIterator::~SampleMapIterator() {}
-
-bool SampleMapIterator::Done() const {
-  return iter_ == end_;
-}
-
-void SampleMapIterator::Next() {
-  DCHECK(!Done());
-  ++iter_;
-  SkipEmptyBuckets();
-}
-
-void SampleMapIterator::Get(Sample* min, Sample* max, Count* count) const {
-  DCHECK(!Done());
-  if (min != NULL)
-    *min = iter_->first;
-  if (max != NULL)
-    *max = iter_->first + 1;
-  if (count != NULL)
-    *count = iter_->second;
-}
-
-void SampleMapIterator::SkipEmptyBuckets() {
-  while (!Done() && iter_->second == 0) {
-    ++iter_;
-  }
-}
-
 }  // namespace base
diff --git a/base/metrics/sample_map.h b/base/metrics/sample_map.h
index da536e3..2f24e1f 100644
--- a/base/metrics/sample_map.h
+++ b/base/metrics/sample_map.h
@@ -20,13 +20,15 @@
 
 namespace base {
 
+// The logic here is similar to that of PersistentSampleMap but with different
+// data structures. Changes here likely need to be duplicated there.
 class BASE_EXPORT SampleMap : public HistogramSamples {
  public:
   SampleMap();
   explicit SampleMap(uint64_t id);
   ~SampleMap() override;
 
-  // HistogramSamples implementation:
+  // HistogramSamples:
   void Accumulate(HistogramBase::Sample value,
                   HistogramBase::Count count) override;
   HistogramBase::Count GetCount(HistogramBase::Sample value) const override;
@@ -34,9 +36,8 @@ class BASE_EXPORT SampleMap : public HistogramSamples {
   scoped_ptr<SampleCountIterator> Iterator() const override;
 
  protected:
-  bool AddSubtractImpl(
-      SampleCountIterator* iter,
-      HistogramSamples::Operator op) override;  // |op| is ADD or SUBTRACT.
+  // Performs arithemetic. |op| is ADD or SUBTRACT.
+  bool AddSubtractImpl(SampleCountIterator* iter, Operator op) override;
 
  private:
   std::map<HistogramBase::Sample, HistogramBase::Count> sample_counts_;
@@ -44,28 +45,6 @@ class BASE_EXPORT SampleMap : public HistogramSamples {
   DISALLOW_COPY_AND_ASSIGN(SampleMap);
 };
 
-class BASE_EXPORT SampleMapIterator : public SampleCountIterator {
- public:
-  typedef std::map<HistogramBase::Sample, HistogramBase::Count>
-      SampleToCountMap;
-
-  explicit SampleMapIterator(const SampleToCountMap& sample_counts);
-  ~SampleMapIterator() override;
-
-  // SampleCountIterator implementation:
-  bool Done() const override;
-  void Next() override;
-  void Get(HistogramBase::Sample* min,
-           HistogramBase::Sample* max,
-           HistogramBase::Count* count) const override;
-
- private:
-  void SkipEmptyBuckets();
-
-  SampleToCountMap::const_iterator iter_;
-  const SampleToCountMap::const_iterator end_;
-};
-
 }  // namespace base
 
 #endif  // BASE_METRICS_SAMPLE_MAP_H_
diff --git a/base/metrics/sparse_histogram.cc b/base/metrics/sparse_histogram.cc
index 485f179..ee18cfa 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -7,6 +7,8 @@
 #include <utility>
 
 #include "base/metrics/metrics_hashes.h"
+#include "base/metrics/persistent_histogram_allocator.h"
+#include "base/metrics/persistent_sample_map.h"
 #include "base/metrics/sample_map.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/pickle.h"
@@ -21,26 +23,75 @@ typedef HistogramBase::Sample Sample;
 // static
 HistogramBase* SparseHistogram::FactoryGet(const std::string& name,
                                            int32_t flags) {
-  HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
+  // Import histograms from known persistent storage. Histograms could have
+  // been added by other processes and they must be fetched and recognized
+  // locally in order to be found by FindHistograms() below. If the persistent
+  // memory segment is not shared between processes, this call does nothing.
+  PersistentHistogramAllocator::ImportGlobalHistograms();
 
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
   if (!histogram) {
-    // To avoid racy destruction at shutdown, the following will be leaked.
-    HistogramBase* tentative_histogram = new SparseHistogram(name);
-    tentative_histogram->SetFlags(flags);
-    histogram =
-        StatisticsRecorder::RegisterOrDeleteDuplicate(tentative_histogram);
+    // Try to create the histogram using a "persistent" allocator. As of
+    // 2016-02-25, the availability of such is controlled by a base::Feature
+    // that is off by default. If the allocator doesn't exist or if
+    // allocating from it fails, code below will allocate the histogram from
+    // the process heap.
+    PersistentMemoryAllocator::Reference histogram_ref = 0;
+    scoped_ptr<HistogramBase> tentative_histogram;
+    PersistentHistogramAllocator* allocator =
+        PersistentHistogramAllocator::GetGlobalAllocator();
+    if (allocator) {
+      tentative_histogram = allocator->AllocateHistogram(
+          SPARSE_HISTOGRAM, name, 0, 0, nullptr, flags, &histogram_ref);
+    }
+
+    // Handle the case where no persistent allocator is present or the
+    // persistent allocation fails (perhaps because it is full).
+    if (!tentative_histogram) {
+      DCHECK(!histogram_ref);  // Should never have been set.
+      DCHECK(!allocator);      // Shouldn't have failed.
+      flags &= ~HistogramBase::kIsPersistent;
+      tentative_histogram.reset(new SparseHistogram(name));
+      tentative_histogram->SetFlags(flags);
+    }
+
+    // Register this histogram with the StatisticsRecorder. Keep a copy of
+    // the pointer value to tell later whether the locally created histogram
+    // was registered or deleted. The type is "void" because it could point
+    // to released memory after the following line.
+    const void* tentative_histogram_ptr = tentative_histogram.get();
+    histogram = StatisticsRecorder::RegisterOrDeleteDuplicate(
+        tentative_histogram.release());
+
+    // Persistent histograms need some follow-up processing.
+    if (histogram_ref) {
+      allocator->FinalizeHistogram(histogram_ref,
+                                   histogram == tentative_histogram_ptr);
+    }
+
     ReportHistogramActivity(*histogram, HISTOGRAM_CREATED);
   } else {
     ReportHistogramActivity(*histogram, HISTOGRAM_LOOKUP);
   }
+
   DCHECK_EQ(SPARSE_HISTOGRAM, histogram->GetHistogramType());
   return histogram;
 }
 
+// static
+scoped_ptr<HistogramBase> SparseHistogram::PersistentCreate(
+    PersistentMemoryAllocator* allocator,
+    const std::string& name,
+    HistogramSamples::Metadata* meta,
+    HistogramSamples::Metadata* logged_meta) {
+  return make_scoped_ptr(
+      new SparseHistogram(allocator, name, meta, logged_meta));
+}
+
 SparseHistogram::~SparseHistogram() {}
 
 uint64_t SparseHistogram::name_hash() const {
-  return samples_.id();
+  return samples_->id();
 }
 
 HistogramType SparseHistogram::GetHistogramType() const {
@@ -66,7 +117,7 @@ void SparseHistogram::AddCount(Sample value, int count) {
   }
   {
     base::AutoLock auto_lock(lock_);
-    samples_.Accumulate(value, count);
+    samples_->Accumulate(value, count);
   }
 
   FindAndRunCallback(value);
@@ -76,29 +127,29 @@ scoped_ptr<HistogramSamples> SparseHistogram::SnapshotSamples() const {
   scoped_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
 
   base::AutoLock auto_lock(lock_);
-  snapshot->Add(samples_);
+  snapshot->Add(*samples_);
   return std::move(snapshot);
 }
 
 scoped_ptr<HistogramSamples> SparseHistogram::SnapshotDelta() {
   scoped_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
   base::AutoLock auto_lock(lock_);
-  snapshot->Add(samples_);
+  snapshot->Add(*samples_);
 
   // Subtract what was previously logged and update that information.
-  snapshot->Subtract(logged_samples_);
-  logged_samples_.Add(*snapshot);
+  snapshot->Subtract(*logged_samples_);
+  logged_samples_->Add(*snapshot);
   return std::move(snapshot);
 }
 
 void SparseHistogram::AddSamples(const HistogramSamples& samples) {
   base::AutoLock auto_lock(lock_);
-  samples_.Add(samples);
+  samples_->Add(samples);
 }
 
 bool SparseHistogram::AddSamplesFromPickle(PickleIterator* iter) {
   base::AutoLock auto_lock(lock_);
-  return samples_.AddFromPickle(iter);
+  return samples_->AddFromPickle(iter);
 }
 
 void SparseHistogram::WriteHTMLGraph(std::string* output) const {
@@ -117,7 +168,28 @@ bool SparseHistogram::SerializeInfoImpl(Pickle* pickle) const {
 
 SparseHistogram::SparseHistogram(const std::string& name)
     : HistogramBase(name),
-      samples_(HashMetricName(name)) {}
+      samples_(new SampleMap(HashMetricName(name))),
+      logged_samples_(new SampleMap(samples_->id())) {}
+
+SparseHistogram::SparseHistogram(PersistentMemoryAllocator* allocator,
+                                 const std::string& name,
+                                 HistogramSamples::Metadata* meta,
+                                 HistogramSamples::Metadata* logged_meta)
+    : HistogramBase(name),
+      // While other histogram types maintain a static vector of values with
+      // sufficient space for both "active" and "logged" samples, with each
+      // SampleVector being given the appropriate half, sparse histograms
+      // have no such initial allocation. Each sample has its own record
+      // attached to a single PersistentSampleMap by a common 64-bit identifier.
+      // Since a sparse histogram has two sample maps (active and logged),
+      // there must be two sets of sample records with diffent IDs. The
+      // "active" samples use, for convenience purposes, an ID matching
+      // that of the histogram while the "logged" samples use that number
+      // plus 1.
+      samples_(new PersistentSampleMap(HashMetricName(name), allocator, meta)),
+      logged_samples_(
+          new PersistentSampleMap(samples_->id() + 1, allocator, logged_meta)) {
+}
 
 HistogramBase* SparseHistogram::DeserializeInfoImpl(PickleIterator* iter) {
   std::string histogram_name;
diff --git a/base/metrics/sparse_histogram.h b/base/metrics/sparse_histogram.h
index 7f36313..b876737 100644
--- a/base/metrics/sparse_histogram.h
+++ b/base/metrics/sparse_histogram.h
@@ -51,6 +51,7 @@ namespace base {
     } while (0)
 
 class HistogramSamples;
+class PersistentMemoryAllocator;
 
 class BASE_EXPORT SparseHistogram : public HistogramBase {
  public:
@@ -58,6 +59,13 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // new one.
   static HistogramBase* FactoryGet(const std::string& name, int32_t flags);
 
+  // Create a histogram using data in persistent storage.
+  static scoped_ptr<HistogramBase> PersistentCreate(
+      PersistentMemoryAllocator* allocator,
+      const std::string& name,
+      HistogramSamples::Metadata* meta,
+      HistogramSamples::Metadata* logged_meta);
+
   ~SparseHistogram() override;
 
   // HistogramBase implementation:
@@ -83,6 +91,11 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // Clients should always use FactoryGet to create SparseHistogram.
   explicit SparseHistogram(const std::string& name);
 
+  SparseHistogram(PersistentMemoryAllocator* allocator,
+                  const std::string& name,
+                  HistogramSamples::Metadata* meta,
+                  HistogramSamples::Metadata* logged_meta);
+
   friend BASE_EXPORT HistogramBase* DeserializeHistogramInfo(
       base::PickleIterator* iter);
   static HistogramBase* DeserializeInfoImpl(base::PickleIterator* iter);
@@ -107,8 +120,8 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // Protects access to |samples_|.
   mutable base::Lock lock_;
 
-  SampleMap samples_;
-  SampleMap logged_samples_;
+  scoped_ptr<HistogramSamples> samples_;
+  scoped_ptr<HistogramSamples> logged_samples_;
 
   DISALLOW_COPY_AND_ASSIGN(SparseHistogram);
 };
diff --git a/base/metrics/sparse_histogram_unittest.cc b/base/metrics/sparse_histogram_unittest.cc
index 7ad5558..5d5dbcb 100644
--- a/base/metrics/sparse_histogram_unittest.cc
+++ b/base/metrics/sparse_histogram_unittest.cc
@@ -9,6 +9,8 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
+#include "base/metrics/persistent_histogram_allocator.h"
+#include "base/metrics/persistent_memory_allocator.h"
 #include "base/metrics/sample_map.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/pickle.h"
@@ -17,17 +19,35 @@
 
 namespace base {
 
-class SparseHistogramTest : public testing::Test {
+// Test parameter indicates if a persistent memory allocator should be used
+// for histogram allocation. False will allocate histograms from the process
+// heap.
+class SparseHistogramTest : public testing::TestWithParam<bool> {
  protected:
+  const int32_t kAllocatorMemorySize = 8 << 20;  // 8 MiB
+
+  SparseHistogramTest() : use_persistent_histogram_allocator_(GetParam()) {}
+
   void SetUp() override {
+    if (use_persistent_histogram_allocator_)
+      CreatePersistentMemoryAllocator();
+
     // Each test will have a clean state (no Histogram / BucketRanges
     // registered).
     InitializeStatisticsRecorder();
   }
 
-  void TearDown() override { UninitializeStatisticsRecorder(); }
+  void TearDown() override {
+    if (allocator_) {
+      ASSERT_FALSE(allocator_->IsFull());
+      ASSERT_FALSE(allocator_->IsCorrupt());
+    }
+    UninitializeStatisticsRecorder();
+    DestroyPersistentMemoryAllocator();
+  }
 
   void InitializeStatisticsRecorder() {
+    StatisticsRecorder::ResetForTesting();
     statistics_recorder_ = new StatisticsRecorder();
   }
 
@@ -36,14 +56,44 @@ class SparseHistogramTest : public testing::Test {
     statistics_recorder_ = NULL;
   }
 
+  void CreatePersistentMemoryAllocator() {
+    // By getting the results-histogram before any persistent allocator
+    // is attached, that histogram is guaranteed not to be stored in
+    // any persistent memory segment (which simplifies some tests).
+    PersistentHistogramAllocator::GetCreateHistogramResultHistogram();
+
+    PersistentHistogramAllocator::CreateGlobalAllocatorOnLocalMemory(
+        kAllocatorMemorySize, 0, "SparseHistogramAllocatorTest");
+    allocator_ =
+        PersistentHistogramAllocator::GetGlobalAllocator()->memory_allocator();
+  }
+
+  void DestroyPersistentMemoryAllocator() {
+    allocator_ = nullptr;
+    PersistentHistogramAllocator::ReleaseGlobalAllocatorForTesting();
+  }
+
   scoped_ptr<SparseHistogram> NewSparseHistogram(const std::string& name) {
     return scoped_ptr<SparseHistogram>(new SparseHistogram(name));
   }
 
+  const bool use_persistent_histogram_allocator_;
+
   StatisticsRecorder* statistics_recorder_;
+  scoped_ptr<char[]> allocator_memory_;
+  PersistentMemoryAllocator* allocator_ = nullptr;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(SparseHistogramTest);
 };
 
-TEST_F(SparseHistogramTest, BasicTest) {
+// Run all HistogramTest cases with both heap and persistent memory.
+INSTANTIATE_TEST_CASE_P(HeapAndPersistent,
+                        SparseHistogramTest,
+                        testing::Bool());
+
+
+TEST_P(SparseHistogramTest, BasicTest) {
   scoped_ptr<SparseHistogram> histogram(NewSparseHistogram("Sparse"));
   scoped_ptr<HistogramSamples> snapshot(histogram->SnapshotSamples());
   EXPECT_EQ(0, snapshot->TotalCount());
@@ -62,7 +112,7 @@ TEST_F(SparseHistogramTest, BasicTest) {
   EXPECT_EQ(1, snapshot2->GetCount(101));
 }
 
-TEST_F(SparseHistogramTest, BasicTestAddCount) {
+TEST_P(SparseHistogramTest, BasicTestAddCount) {
   scoped_ptr<SparseHistogram> histogram(NewSparseHistogram("Sparse"));
   scoped_ptr<HistogramSamples> snapshot(histogram->SnapshotSamples());
   EXPECT_EQ(0, snapshot->TotalCount());
@@ -81,7 +131,7 @@ TEST_F(SparseHistogramTest, BasicTestAddCount) {
   EXPECT_EQ(25, snapshot2->GetCount(101));
 }
 
-TEST_F(SparseHistogramTest, AddCount_LargeValuesDontOverflow) {
+TEST_P(SparseHistogramTest, AddCount_LargeValuesDontOverflow) {
   scoped_ptr<SparseHistogram> histogram(NewSparseHistogram("Sparse"));
   scoped_ptr<HistogramSamples> snapshot(histogram->SnapshotSamples());
   EXPECT_EQ(0, snapshot->TotalCount());
@@ -101,7 +151,7 @@ TEST_F(SparseHistogramTest, AddCount_LargeValuesDontOverflow) {
   EXPECT_EQ(55250000000LL, snapshot2->sum());
 }
 
-TEST_F(SparseHistogramTest, MacroBasicTest) {
+TEST_P(SparseHistogramTest, MacroBasicTest) {
   UMA_HISTOGRAM_SPARSE_SLOWLY("Sparse", 100);
   UMA_HISTOGRAM_SPARSE_SLOWLY("Sparse", 200);
   UMA_HISTOGRAM_SPARSE_SLOWLY("Sparse", 100);
@@ -114,8 +164,11 @@ TEST_F(SparseHistogramTest, MacroBasicTest) {
 
   EXPECT_EQ(SPARSE_HISTOGRAM, sparse_histogram->GetHistogramType());
   EXPECT_EQ("Sparse", sparse_histogram->histogram_name());
-  EXPECT_EQ(HistogramBase::kUmaTargetedHistogramFlag,
-            sparse_histogram->flags());
+  EXPECT_EQ(
+      HistogramBase::kUmaTargetedHistogramFlag |
+          (use_persistent_histogram_allocator_ ? HistogramBase::kIsPersistent
+                                               : 0),
+      sparse_histogram->flags());
 
   scoped_ptr<HistogramSamples> samples = sparse_histogram->SnapshotSamples();
   EXPECT_EQ(3, samples->TotalCount());
@@ -123,7 +176,7 @@ TEST_F(SparseHistogramTest, MacroBasicTest) {
   EXPECT_EQ(1, samples->GetCount(200));
 }
 
-TEST_F(SparseHistogramTest, MacroInLoopTest) {
+TEST_P(SparseHistogramTest, MacroInLoopTest) {
   // Unlike the macros in histogram.h, SparseHistogram macros can have a
   // variable as histogram name.
   for (int i = 0; i < 2; i++) {
@@ -141,7 +194,7 @@ TEST_F(SparseHistogramTest, MacroInLoopTest) {
               ("Sparse2" == name1 && "Sparse1" == name2));
 }
 
-TEST_F(SparseHistogramTest, Serialize) {
+TEST_P(SparseHistogramTest, Serialize) {
   scoped_ptr<SparseHistogram> histogram(NewSparseHistogram("Sparse"));
   histogram->SetFlags(HistogramBase::kIPCSerializationSourceFlag);
 
@@ -166,4 +219,64 @@ TEST_F(SparseHistogramTest, Serialize) {
   EXPECT_FALSE(iter.SkipBytes(1));
 }
 
+TEST_P(SparseHistogramTest, FactoryTime) {
+  const int kTestCreateCount = 1 << 10;  // Must be power-of-2.
+  const int kTestLookupCount = 100000;
+  const int kTestAddCount = 100000;
+
+  // Create all histogram names in advance for accurate timing below.
+  std::vector<std::string> histogram_names;
+  for (int i = 0; i < kTestCreateCount; ++i) {
+    histogram_names.push_back(
+        StringPrintf("TestHistogram.%d", i % kTestCreateCount));
+  }
+
+  // Calculate cost of creating histograms.
+  TimeTicks create_start = TimeTicks::Now();
+  for (int i = 0; i < kTestCreateCount; ++i)
+    SparseHistogram::FactoryGet(histogram_names[i], HistogramBase::kNoFlags);
+  TimeDelta create_ticks = TimeTicks::Now() - create_start;
+  int64_t create_ms = create_ticks.InMilliseconds();
+
+  VLOG(1) << kTestCreateCount << " histogram creations took " << create_ms
+          << "ms or about "
+          << (create_ms * 1000000) / kTestCreateCount
+          << "ns each.";
+
+  // Calculate cost of looking up existing histograms.
+  TimeTicks lookup_start = TimeTicks::Now();
+  for (int i = 0; i < kTestLookupCount; ++i) {
+    // 6007 is co-prime with kTestCreateCount and so will do lookups in an
+    // order less likely to be cacheable (but still hit them all) should the
+    // underlying storage use the exact histogram name as the key.
+    const int i_mult = 6007;
+    static_assert(i_mult < INT_MAX / kTestCreateCount, "Multiplier too big");
+    int index = (i * i_mult) & (kTestCreateCount - 1);
+    SparseHistogram::FactoryGet(histogram_names[index],
+                                HistogramBase::kNoFlags);
+  }
+  TimeDelta lookup_ticks = TimeTicks::Now() - lookup_start;
+  int64_t lookup_ms = lookup_ticks.InMilliseconds();
+
+  VLOG(1) << kTestLookupCount << " histogram lookups took " << lookup_ms
+          << "ms or about "
+          << (lookup_ms * 1000000) / kTestLookupCount
+          << "ns each.";
+
+  // Calculate cost of accessing histograms.
+  HistogramBase* histogram =
+      SparseHistogram::FactoryGet(histogram_names[0], HistogramBase::kNoFlags);
+  ASSERT_TRUE(histogram);
+  TimeTicks add_start = TimeTicks::Now();
+  for (int i = 0; i < kTestAddCount; ++i)
+    histogram->Add(i & 127);
+  TimeDelta add_ticks = TimeTicks::Now() - add_start;
+  int64_t add_ms = add_ticks.InMilliseconds();
+
+  VLOG(1) << kTestAddCount << " histogram adds took " << add_ms
+          << "ms or about "
+          << (add_ms * 1000000) / kTestAddCount
+          << "ns each.";
+}
+
 }  // namespace base