Improve efficiency of persistent sparse histograms.

base/metrics/persistent_sample_map.h

 // 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 <memory>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/persistent_memory_allocator.h"
 
 namespace base {
 
+class PersistentHistogramAllocator;
+class PersistentSampleMapRecords;
+class PersistentSparseHistogramDataManager;
+
 // 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:
+  // Constructs a persistent sample map using any of a variety of persistent
+  // data sources. Really, the first two are just convenience methods for
+  // getting at the PersistentSampleMapRecords object for the specified |id|.
+  // The source objects must live longer than this object.
   PersistentSampleMap(uint64_t id,
-                      PersistentMemoryAllocator* allocator,
+                      PersistentHistogramAllocator* allocator,
                       Metadata* meta);
+  PersistentSampleMap(uint64_t id,
+                      PersistentSparseHistogramDataManager* manager,
+                      Metadata* meta);
+  PersistentSampleMap(uint64_t id,
+                      PersistentSampleMapRecords* records,
+                      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;
   std::unique_ptr<SampleCountIterator> Iterator() const override;
 
+  // Uses a persistent-memory |iterator| to locate and return information about
+  // the next record holding information for a PersistentSampleMap. The record
+  // could be for any Map so return the |sample_map_id| as well.
+  static PersistentMemoryAllocator::Reference GetNextPersistentRecord(
+      PersistentMemoryAllocator::Iterator& iterator,
+      uint64_t* sample_map_id);
+
+  // Creates a new record in an |allocator| storing count information for a
+  // specific sample |value| of a histogram with the given |sample_map_id|.
+  static PersistentMemoryAllocator::Reference CreatePersistentRecord(
+      PersistentMemoryAllocator* allocator,
+      uint64_t sample_map_id,
+      HistogramBase::Sample value);
+
  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_|.
+  // for the actual Count numbers is owned by the |records_| object and its
+  // underlying 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_;
+  // The object that manages records inside persistent memory. This is owned
+  // externally (typically by a PersistentHistogramAllocator) and is expected
+  // to live beyond the life of this object.
+  PersistentSampleMapRecords* records_;
 
   DISALLOW_COPY_AND_ASSIGN(PersistentSampleMap);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_PERSISTENT_SAMPLE_MAP_H_