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// 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.
#include <stddef.h>
#include <stdint.h>
#include <limits>
#include <memory>
#include "base/atomicops.h"
#include "base/macros.h"
#include "base/metrics/histogram_base.h"
namespace base {
class Pickle;
class PickleIterator;
class SampleCountIterator;
// HistogramSamples is a container storing all samples of a histogram. All
// elements must be of a fixed width to ensure 32/64-bit interoperability.
// If this structure changes, bump the version number for kTypeIdHistogram
// in
// Note that though these samples are individually consistent (through the use
// of atomic operations on the counts), there is only "eventual consistency"
// overall when multiple threads are accessing this data. That means that the
// sum, redundant-count, etc. could be momentarily out-of-sync with the stored
// counts but will settle to a consistent "steady state" once all threads have
// exited this code.
class BASE_EXPORT HistogramSamples {
// A single bucket and count. To fit within a single atomic on 32-bit build
// architectures, both |bucket| and |count| are limited in size to 16 bits.
// This limits the functionality somewhat but if an entry can't fit then
// the full array of samples can be allocated and used.
struct SingleSample {
uint16_t bucket;
uint16_t count;
// A structure for managing an atomic single sample. Because this is generally
// used in association with other atomic values, the defined methods use
// acquire/release operations to guarantee ordering with outside values.
union BASE_EXPORT AtomicSingleSample {
AtomicSingleSample() : as_atomic(0) {}
AtomicSingleSample(subtle::Atomic32 rhs) : as_atomic(rhs) {}
// Returns the single sample in an atomic manner. This in an "acquire"
// load. The returned sample isn't shared and thus its fields can be safely
// accessed.
SingleSample Load() const;
// Extracts the single sample in an atomic manner. If |disable| is true
// then this object will be set so it will never accumulate another value.
// This is "no barrier" so doesn't enforce ordering with other atomic ops.
SingleSample Extract(bool disable);
// Adds a given count to the held bucket. If not possible, it returns false
// and leaves the parts unchanged. Once extracted/disabled, this always
// returns false. This in an "acquire/release" operation.
bool Accumulate(size_t bucket, HistogramBase::Count count);
// Returns if the sample has been "disabled" (via Extract) and thus not
// allowed to accept further accumulation.
bool IsDisabled() const;
// union field: The actual sample bucket and count.
SingleSample as_parts;
// union field: The sample as an atomic value. Atomic64 would provide
// more flexibility but isn't available on all builds. This can hold a
// special, internal "disabled" value indicating that it must not accept
// further accumulation.
subtle::Atomic32 as_atomic;
// A structure of information about the data, common to all sample containers.
// Because of how this is used in persistent memory, it must be a POD object
// that makes sense when initialized to all zeros.
struct Metadata {
// Expected size for 32/64-bit check.
static constexpr size_t kExpectedInstanceSize = 24;
// Initialized when the sample-set is first created with a value provided
// by the caller. It is generally used to identify the sample-set across
// threads and processes, though not necessarily uniquely as it is possible
// to have multiple sample-sets representing subsets of the data.
uint64_t id;
// The sum of all the entries, effectivly the sum(sample * count) for
// all samples. Despite being atomic, no guarantees are made on the
// accuracy of this value; there may be races during histogram
// accumulation and snapshotting that we choose to accept. It should
// be treated as approximate.
#ifdef ARCH_CPU_64_BITS
subtle::Atomic64 sum;
// 32-bit systems don't have atomic 64-bit operations. Use a basic type
// and don't worry about "shearing".
int64_t sum;
// A "redundant" count helps identify memory corruption. It redundantly
// stores the total number of samples accumulated in the histogram. We
// can compare this count to the sum of the counts (TotalCount() function),
// and detect problems. Note, depending on the implementation of different
// histogram types, there might be races during histogram accumulation
// and snapshotting that we choose to accept. In this case, the tallies
// might mismatch even when no memory corruption has happened.
HistogramBase::AtomicCount redundant_count;
// A single histogram value and associated count. This allows histograms
// that typically report only a single value to not require full storage
// to be allocated.
AtomicSingleSample single_sample; // 32 bits
// Because structures held in persistent memory must be POD, there can be no
// default constructor to clear the fields. This derived class exists just
// to clear them when being allocated on the heap.
struct BASE_EXPORT LocalMetadata : Metadata {
HistogramSamples(uint64_t id, Metadata* meta);
virtual ~HistogramSamples();
virtual void Accumulate(HistogramBase::Sample value,
HistogramBase::Count count) = 0;
virtual HistogramBase::Count GetCount(HistogramBase::Sample value) const = 0;
virtual HistogramBase::Count TotalCount() const = 0;
virtual void Add(const HistogramSamples& other);
// Add from serialized samples.
virtual bool AddFromPickle(PickleIterator* iter);
virtual void Subtract(const HistogramSamples& other);
virtual std::unique_ptr<SampleCountIterator> Iterator() const = 0;
virtual void Serialize(Pickle* pickle) const;
// Accessor fuctions.
uint64_t id() const { return meta_->id; }
int64_t sum() const {
#ifdef ARCH_CPU_64_BITS
return subtle::NoBarrier_Load(&meta_->sum);
return meta_->sum;
HistogramBase::Count redundant_count() const {
return subtle::NoBarrier_Load(&meta_->redundant_count);
enum NegativeSampleReason {
// Based on |op| type, add or subtract sample counts data from the iterator.
enum Operator { ADD, SUBTRACT };
virtual bool AddSubtractImpl(SampleCountIterator* iter, Operator op) = 0;
// Accumulates to the embedded single-sample field if possible. Returns true
// on success, false otherwise. Sum and redundant-count are also updated in
// the success case.
bool AccumulateSingleSample(HistogramBase::Sample value,
HistogramBase::Count count,
size_t bucket);
// Atomically adjust the sum and redundant-count.
void IncreaseSumAndCount(int64_t sum, HistogramBase::Count count);
// Record a negative-sample observation and the reason why.
void RecordNegativeSample(NegativeSampleReason reason,
HistogramBase::Count increment);
AtomicSingleSample& single_sample() { return meta_->single_sample; }
const AtomicSingleSample& single_sample() const {
return meta_->single_sample;
Metadata* meta() { return meta_; }
// Depending on derived class meta values can come from local stoarge or
// external storage in which case HistogramSamples class cannot take ownership
// of Metadata*.
Metadata* meta_;
class BASE_EXPORT SampleCountIterator {
virtual ~SampleCountIterator();
virtual bool Done() const = 0;
virtual void Next() = 0;
// Get the sample and count at current position.
// |min| |max| and |count| can be NULL if the value is not of interest.
// Note: |max| is int64_t because histograms support logged values in the
// full int32_t range and bucket max is exclusive, so it needs to support
// values up to MAXINT32+1.
// Requires: !Done();
virtual void Get(HistogramBase::Sample* min,
int64_t* max,
HistogramBase::Count* count) const = 0;
static_assert(std::numeric_limits<HistogramBase::Sample>::max() <
"Get() |max| must be able to hold Histogram::Sample max + 1");
// Get the index of current histogram bucket.
// For histograms that don't use predefined buckets, it returns false.
// Requires: !Done();
virtual bool GetBucketIndex(size_t* index) const;
class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
SingleSampleIterator(HistogramBase::Sample min,
int64_t max,
HistogramBase::Count count);
SingleSampleIterator(HistogramBase::Sample min,
int64_t max,
HistogramBase::Count count,
size_t bucket_index);
~SingleSampleIterator() override;
// SampleCountIterator:
bool Done() const override;
void Next() override;
void Get(HistogramBase::Sample* min,
int64_t* max,
HistogramBase::Count* count) const override;
// SampleVector uses predefined buckets so iterator can return bucket index.
bool GetBucketIndex(size_t* index) const override;
// Information about the single value to return.
const HistogramBase::Sample min_;
const int64_t max_;
const size_t bucket_index_;
HistogramBase::Count count_;
} // namespace base