Support "shared" histograms between processes.

base/metrics/histogram_base.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.
 
 #ifndef BASE_METRICS_HISTOGRAM_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <stdint.h>
 
 #include <string>
 #include <vector>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/memory/persistent_memory_allocator.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_piece.h"
 #include "base/time/time.h"
 
 namespace base {
 
+class BucketRanges;
 class DictionaryValue;
 class HistogramBase;
 class HistogramSamples;
 class ListValue;
 class Pickle;
 class PickleIterator;
 
 ////////////////////////////////////////////////////////////////////////////////
 // These enums are used to facilitate deserialization of histograms from other
 // processes into the browser. If you create another class that inherits from
@@ skipping 40 matching lines @@
     // aggregation should not take place (as we would be aggregating back into
     // the source histogram!).
     kIPCSerializationSourceFlag = 0x10,
 
     // Indicates that a callback exists for when a new sample is recorded on
     // this histogram. We store this as a flag with the histogram since
     // histograms can be in performance critical code, and this allows us
     // to shortcut looking up the callback if it doesn't exist.
     kCallbackExists = 0x20,
 
+    // Indicates that the histogram is held in "persistent" memory and may
+    // be accessible between processes. This is only possible if such a
+    // memory segment has been created/attached, used to create a Persistent-
+    // MemoryAllocator, and that loaded into the Histogram module before this
+    // histogram is created.
+    kIsPersistent = 0x40,
+
     // Only for Histogram and its sub classes: fancy bucket-naming support.
     kHexRangePrintingFlag = 0x8000,
   };
 
   // Histogram data inconsistency types.
   enum Inconsistency {
     NO_INCONSISTENCIES = 0x0,
     RANGE_CHECKSUM_ERROR = 0x1,
     BUCKET_ORDER_ERROR = 0x2,
     COUNT_HIGH_ERROR = 0x4,
@@ skipping 59 matching lines @@
 
   // The following methods provide graphical histogram displays.
   virtual void WriteHTMLGraph(std::string* output) const = 0;
   virtual void WriteAscii(std::string* output) const = 0;
 
   // Produce a JSON representation of the histogram. This is implemented with
   // the help of GetParameters and GetCountAndBucketData; overwrite them to
   // customize the output.
   void WriteJSON(std::string* output) const;
 
+  // Access a PersistentMemoryAllocator for storing histograms in space that
+  // can be persisted or shared between processes. There is only ever one
+  // allocator for all such histograms created by a single process though one
+  // process may access the histograms created by other processes if it has a
+  // handle on its memory segment. This takes ownership of the object and
+  // should not be changed without great care as it is likely that there will
+  // be pointers to data held in that space.
+  static void SetDefaultPersistentMemoryAllocator(PersistentMemoryAllocator* a);
+  static PersistentMemoryAllocator* GetDefaultPersistentMemoryAllocator() {
+    return allocator_.get();
+  }
+
+  // This access to the persistent allocator is only for testing; it extracts
+  // the current allocator completely. This allows easy creation of histograms
+  // within persistent memory segments which can then be extracted and used
+  // in other ways.
+  static PersistentMemoryAllocator* ReleaseDefaultPersistentMemoryAllocator() {
+    return allocator_.release();
+  };
+
+  // Recreate a Histogram from data held in persistent memory. Though this
+  // object will be local to the current process, the sample data will be
+  // shared with all other threads referencing it. This method takes a |ref|
+  // to the top- level histogram data and the |allocator| on which it is found.
+  // This method will return nullptr if any problem is detected with the data.
+  // The |allocator| may or may not be the same as the PersistentMemoryAllocator
+  // set for general use so that this method can be used to extract Histograms
+  // from persistent memory segments other than the default place that this
+  // process is creating its own histograms. The caller must take ownership of
+  // the returned object and destroy it when no longer needed.
+  static HistogramBase* GetPersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      int32_t ref);
+
+  // Get the next histogram in persistent data based on iterator. The caller
+  // must take ownership of the returned object and destroy it when no longer
+  // needed.
+  static HistogramBase* GetNextPersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      PersistentMemoryAllocator::Iterator* iter);

[Alexei Svitkine (slow), 2015/12/04 18:27:36]

Can you put these in a separate file? HistogramBase is an abstract base class
and shouldn't be concerned with how histograms are allocated.

[bcwhite, 2015/12/07 23:27:19]

Done.

+
  protected:
+  struct PersistentHistogramData;
+
+  // Global allocator used when histograms should be persistent.
+  static scoped_ptr<PersistentMemoryAllocator> allocator_;
+
   // Subclasses should implement this function to make SerializeInfo work.
   virtual bool SerializeInfoImpl(base::Pickle* pickle) const = 0;
 
   // Writes information about the construction parameters in |params|.
   virtual void GetParameters(DictionaryValue* params) const = 0;
 
   // Writes information about the current (non-empty) buckets and their sample
   // counts to |buckets|, the total sample count to |count| and the total sum
   // to |sum|.
   virtual void GetCountAndBucketData(Count* count,
@@ skipping 11 matching lines @@
   // Write textual description of the bucket contents (relative to histogram).
   // Output is the count in the buckets, as well as the percentage.
   void WriteAsciiBucketValue(Count current,
                              double scaled_sum,
                              std::string* output) const;
 
   // Retrieves the callback for this histogram, if one exists, and runs it
   // passing |sample| as the parameter.
   void FindAndRunCallback(Sample sample) const;
 
+  // Create a histogram based on persistent data.
+  static HistogramBase* CreatePersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      PersistentHistogramData* histogram_data);
+
+  // Allocate a new persistent histogram. This does *not* make the object
+  // iterable in the allocator; call MakeIterable(ref) directly if that is
+  // desired.
+  static HistogramBase* AllocatePersistentHistogram(
+      PersistentMemoryAllocator* allocator,
+      HistogramType histogram_type,
+      const std::string& name,
+      int minimum,
+      int maximum,
+      const BucketRanges* bucket_ranges,
+      int32 flags,
+      PersistentMemoryAllocator::Reference* ref_ptr);
+
+  // Import new histograms from attached PersistentMemoryAllocator. It's
+  // possible for other processes to create histograms in the attached memory
+  // segment; this adds those to the internal list of known histograms to
+  // avoid creating duplicates that would have to merged during reporting.
+  // Every call to this method resumes from the last entry it saw so it costs
+  // nothing if nothing new has been added.
+  static void ImportPersistentHistograms();
+
  private:
   const std::string histogram_name_;
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_