268 lines
10 KiB
C
268 lines
10 KiB
C
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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_METRICS_HISTOGRAM_SAMPLES_H_
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#define BASE_METRICS_HISTOGRAM_SAMPLES_H_
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#include <stddef.h>
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#include <stdint.h>
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#include <limits>
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#include <memory>
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#include "base/atomicops.h"
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#include "base/macros.h"
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#include "base/metrics/histogram_base.h"
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namespace base {
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class Pickle;
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class PickleIterator;
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class SampleCountIterator;
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// HistogramSamples is a container storing all samples of a histogram. All
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// elements must be of a fixed width to ensure 32/64-bit interoperability.
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// If this structure changes, bump the version number for kTypeIdHistogram
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// in persistent_histogram_allocator.cc.
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//
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// Note that though these samples are individually consistent (through the use
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// of atomic operations on the counts), there is only "eventual consistency"
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// overall when multiple threads are accessing this data. That means that the
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// sum, redundant-count, etc. could be momentarily out-of-sync with the stored
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// counts but will settle to a consistent "steady state" once all threads have
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// exited this code.
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class BASE_EXPORT HistogramSamples {
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public:
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// A single bucket and count. To fit within a single atomic on 32-bit build
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// architectures, both |bucket| and |count| are limited in size to 16 bits.
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// This limits the functionality somewhat but if an entry can't fit then
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// the full array of samples can be allocated and used.
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struct SingleSample {
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uint16_t bucket;
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uint16_t count;
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};
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// A structure for managing an atomic single sample. Because this is generally
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// used in association with other atomic values, the defined methods use
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// acquire/release operations to guarantee ordering with outside values.
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union BASE_EXPORT AtomicSingleSample {
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AtomicSingleSample() : as_atomic(0) {}
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AtomicSingleSample(subtle::Atomic32 rhs) : as_atomic(rhs) {}
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// Returns the single sample in an atomic manner. This in an "acquire"
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// load. The returned sample isn't shared and thus its fields can be safely
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// accessed.
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SingleSample Load() const;
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// Extracts the single sample in an atomic manner. If |disable| is true
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// then this object will be set so it will never accumulate another value.
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// This is "no barrier" so doesn't enforce ordering with other atomic ops.
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SingleSample Extract(bool disable);
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// Adds a given count to the held bucket. If not possible, it returns false
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// and leaves the parts unchanged. Once extracted/disabled, this always
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// returns false. This in an "acquire/release" operation.
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bool Accumulate(size_t bucket, HistogramBase::Count count);
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// Returns if the sample has been "disabled" (via Extract) and thus not
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// allowed to accept further accumulation.
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bool IsDisabled() const;
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private:
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// union field: The actual sample bucket and count.
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SingleSample as_parts;
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// union field: The sample as an atomic value. Atomic64 would provide
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// more flexibility but isn't available on all builds. This can hold a
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// special, internal "disabled" value indicating that it must not accept
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// further accumulation.
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subtle::Atomic32 as_atomic;
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};
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// A structure of information about the data, common to all sample containers.
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// Because of how this is used in persistent memory, it must be a POD object
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// that makes sense when initialized to all zeros.
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struct Metadata {
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// Expected size for 32/64-bit check.
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static constexpr size_t kExpectedInstanceSize = 24;
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// Initialized when the sample-set is first created with a value provided
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// by the caller. It is generally used to identify the sample-set across
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// threads and processes, though not necessarily uniquely as it is possible
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// to have multiple sample-sets representing subsets of the data.
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uint64_t id;
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// The sum of all the entries, effectivly the sum(sample * count) for
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// all samples. Despite being atomic, no guarantees are made on the
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// accuracy of this value; there may be races during histogram
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// accumulation and snapshotting that we choose to accept. It should
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// be treated as approximate.
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#ifdef ARCH_CPU_64_BITS
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subtle::Atomic64 sum;
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#else
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// 32-bit systems don't have atomic 64-bit operations. Use a basic type
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// and don't worry about "shearing".
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int64_t sum;
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#endif
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// A "redundant" count helps identify memory corruption. It redundantly
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// stores the total number of samples accumulated in the histogram. We
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// can compare this count to the sum of the counts (TotalCount() function),
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// and detect problems. Note, depending on the implementation of different
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// histogram types, there might be races during histogram accumulation
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// and snapshotting that we choose to accept. In this case, the tallies
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// might mismatch even when no memory corruption has happened.
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HistogramBase::AtomicCount redundant_count;
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// A single histogram value and associated count. This allows histograms
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// that typically report only a single value to not require full storage
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// to be allocated.
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AtomicSingleSample single_sample; // 32 bits
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};
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// Because structures held in persistent memory must be POD, there can be no
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// default constructor to clear the fields. This derived class exists just
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// to clear them when being allocated on the heap.
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struct BASE_EXPORT LocalMetadata : Metadata {
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LocalMetadata();
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};
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HistogramSamples(uint64_t id, Metadata* meta);
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virtual ~HistogramSamples();
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virtual void Accumulate(HistogramBase::Sample value,
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HistogramBase::Count count) = 0;
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virtual HistogramBase::Count GetCount(HistogramBase::Sample value) const = 0;
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virtual HistogramBase::Count TotalCount() const = 0;
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virtual void Add(const HistogramSamples& other);
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// Add from serialized samples.
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virtual bool AddFromPickle(PickleIterator* iter);
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virtual void Subtract(const HistogramSamples& other);
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virtual std::unique_ptr<SampleCountIterator> Iterator() const = 0;
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virtual void Serialize(Pickle* pickle) const;
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// Accessor fuctions.
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uint64_t id() const { return meta_->id; }
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int64_t sum() const {
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#ifdef ARCH_CPU_64_BITS
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return subtle::NoBarrier_Load(&meta_->sum);
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#else
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return meta_->sum;
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#endif
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}
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HistogramBase::Count redundant_count() const {
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return subtle::NoBarrier_Load(&meta_->redundant_count);
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}
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protected:
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enum NegativeSampleReason {
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SAMPLES_HAVE_LOGGED_BUT_NOT_SAMPLE,
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SAMPLES_SAMPLE_LESS_THAN_LOGGED,
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SAMPLES_ADDED_NEGATIVE_COUNT,
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SAMPLES_ADD_WENT_NEGATIVE,
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SAMPLES_ADD_OVERFLOW,
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SAMPLES_ACCUMULATE_NEGATIVE_COUNT,
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SAMPLES_ACCUMULATE_WENT_NEGATIVE,
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DEPRECATED_SAMPLES_ACCUMULATE_OVERFLOW,
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SAMPLES_ACCUMULATE_OVERFLOW,
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MAX_NEGATIVE_SAMPLE_REASONS
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};
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// Based on |op| type, add or subtract sample counts data from the iterator.
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enum Operator { ADD, SUBTRACT };
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virtual bool AddSubtractImpl(SampleCountIterator* iter, Operator op) = 0;
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// Accumulates to the embedded single-sample field if possible. Returns true
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// on success, false otherwise. Sum and redundant-count are also updated in
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// the success case.
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bool AccumulateSingleSample(HistogramBase::Sample value,
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HistogramBase::Count count,
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size_t bucket);
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// Atomically adjust the sum and redundant-count.
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void IncreaseSumAndCount(int64_t sum, HistogramBase::Count count);
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// Record a negative-sample observation and the reason why.
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void RecordNegativeSample(NegativeSampleReason reason,
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HistogramBase::Count increment);
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AtomicSingleSample& single_sample() { return meta_->single_sample; }
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const AtomicSingleSample& single_sample() const {
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return meta_->single_sample;
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}
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Metadata* meta() { return meta_; }
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private:
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// Depending on derived class meta values can come from local stoarge or
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// external storage in which case HistogramSamples class cannot take ownership
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// of Metadata*.
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Metadata* meta_;
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DISALLOW_COPY_AND_ASSIGN(HistogramSamples);
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};
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class BASE_EXPORT SampleCountIterator {
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public:
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virtual ~SampleCountIterator();
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virtual bool Done() const = 0;
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virtual void Next() = 0;
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// Get the sample and count at current position.
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// |min| |max| and |count| can be NULL if the value is not of interest.
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// Note: |max| is int64_t because histograms support logged values in the
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// full int32_t range and bucket max is exclusive, so it needs to support
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// values up to MAXINT32+1.
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// Requires: !Done();
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virtual void Get(HistogramBase::Sample* min,
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int64_t* max,
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HistogramBase::Count* count) const = 0;
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static_assert(std::numeric_limits<HistogramBase::Sample>::max() <
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std::numeric_limits<int64_t>::max(),
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"Get() |max| must be able to hold Histogram::Sample max + 1");
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// Get the index of current histogram bucket.
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// For histograms that don't use predefined buckets, it returns false.
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// Requires: !Done();
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virtual bool GetBucketIndex(size_t* index) const;
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};
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class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
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public:
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SingleSampleIterator(HistogramBase::Sample min,
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int64_t max,
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HistogramBase::Count count);
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SingleSampleIterator(HistogramBase::Sample min,
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int64_t max,
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HistogramBase::Count count,
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size_t bucket_index);
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~SingleSampleIterator() override;
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// SampleCountIterator:
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bool Done() const override;
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void Next() override;
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void Get(HistogramBase::Sample* min,
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int64_t* max,
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HistogramBase::Count* count) const override;
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// SampleVector uses predefined buckets so iterator can return bucket index.
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bool GetBucketIndex(size_t* index) const override;
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private:
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// Information about the single value to return.
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const HistogramBase::Sample min_;
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const int64_t max_;
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const size_t bucket_index_;
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HistogramBase::Count count_;
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};
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} // namespace base
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#endif // BASE_METRICS_HISTOGRAM_SAMPLES_H_
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