De-duplicate ranges information in persistent memory.

base/metrics/bucket_ranges.h

 // 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.
 //
 // BucketRanges stores the vector of ranges that delimit what samples are
 // tallied in the corresponding buckets of a histogram. Histograms that have
 // same ranges for all their corresponding buckets should share the same
 // BucketRanges object.
 //
 // E.g. A 5 buckets LinearHistogram with 1 as minimal value and 4 as maximal
 // value will need a BucketRanges with 6 ranges:
 // 0, 1, 2, 3, 4, INT_MAX
 //
 // TODO(kaiwang): Currently we keep all negative values in 0~1 bucket. Consider
 // changing 0 to INT_MIN.
 
 #ifndef BASE_METRICS_BUCKET_RANGES_H_
 #define BASE_METRICS_BUCKET_RANGES_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <vector>
 
 #include <limits.h>
 
+#include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
 
 namespace base {
 
 class BASE_EXPORT BucketRanges {
  public:
   typedef std::vector<HistogramBase::Sample> Ranges;
 
@@ skipping 14 matching lines @@
 
   // Checksum methods to verify whether the ranges are corrupted (e.g. bad
   // memory access).
   uint32_t CalculateChecksum() const;
   bool HasValidChecksum() const;
   void ResetChecksum();
 
   // Return true iff |other| object has same ranges_ as |this| object's ranges_.
   bool Equals(const BucketRanges* other) const;
 
+  // Set and get a reference into persistent memory where this bucket data
+  // can be found (and re-used). These calls are internally atomic with no
+  // safety against overwriting an existing value since though it is wasteful
+  // to have multiple identical persistent records, it is still safe.
+  void set_persistent_reference(uint32_t ref) const {
+    subtle::NoBarrier_Store(&persistent_reference_, ref);
+  }
+  uint32_t persistent_reference() const {
+    return subtle::NoBarrier_Load(&persistent_reference_);
+  }
+
  private:
   // A monotonically increasing list of values which determine which bucket to
   // put a sample into.  For each index, show the smallest sample that can be
   // added to the corresponding bucket.
   Ranges ranges_;
 
   // Checksum for the conntents of ranges_.  Used to detect random over-writes
   // of our data, and to quickly see if some other BucketRanges instance is
   // possibly Equal() to this instance.
   // TODO(kaiwang): Consider change this to uint64_t. Because we see a lot of
   // noise on UMA dashboard.
   uint32_t checksum_;
 
+  // A reference into a global PersistentMemoryAllocator where the ranges
+  // information is stored. This allows for the record to be created once and
+  // re-used simply by having all histograms with the same ranges use the
+  // same reference.
+  mutable subtle::Atomic32 persistent_reference_ = 0;
+
   DISALLOW_COPY_AND_ASSIGN(BucketRanges);
 };
 
 //////////////////////////////////////////////////////////////////////////////
 // Expose only for test.
 BASE_EXPORT extern const uint32_t kCrcTable[256];
 
 }  // namespace base
 
 #endif  // BASE_METRICS_BUCKET_RANGES_H_