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rocksdb/monitoring/histogram.cc

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#include "monitoring/histogram.h"
#include <stdio.h>
#include <cassert>
#include <cinttypes>
#include <cmath>
#include "port/port.h"
#include "util/cast_util.h"
namespace ROCKSDB_NAMESPACE {
HistogramBucketMapper::HistogramBucketMapper() {
// If you change this, you also need to change
// size of array buckets_ in HistogramImpl
bucketValues_ = {1, 2};
valueIndexMap_ = {{1, 0}, {2, 1}};
double bucket_val = static_cast<double>(bucketValues_.back());
while ((bucket_val = 1.5 * bucket_val) <= static_cast<double>(port::kMaxUint64)) {
bucketValues_.push_back(static_cast<uint64_t>(bucket_val));
// Extracts two most significant digits to make histogram buckets more
// human-readable. E.g., 172 becomes 170.
uint64_t pow_of_ten = 1;
while (bucketValues_.back() / 10 > 10) {
bucketValues_.back() /= 10;
pow_of_ten *= 10;
}
bucketValues_.back() *= pow_of_ten;
valueIndexMap_[bucketValues_.back()] = bucketValues_.size() - 1;
}
maxBucketValue_ = bucketValues_.back();
minBucketValue_ = bucketValues_.front();
}
size_t HistogramBucketMapper::IndexForValue(const uint64_t value) const {
if (value >= maxBucketValue_) {
return bucketValues_.size() - 1;
} else if ( value >= minBucketValue_ ) {
std::map<uint64_t, uint64_t>::const_iterator lowerBound =
valueIndexMap_.lower_bound(value);
if (lowerBound != valueIndexMap_.end()) {
return static_cast<size_t>(lowerBound->second);
} else {
return 0;
}
} else {
return 0;
}
}
namespace {
const HistogramBucketMapper bucketMapper;
}
HistogramStat::HistogramStat()
: num_buckets_(bucketMapper.BucketCount()) {
assert(num_buckets_ == sizeof(buckets_) / sizeof(*buckets_));
Clear();
}
void HistogramStat::Clear() {
min_.store(bucketMapper.LastValue(), std::memory_order_relaxed);
max_.store(0, std::memory_order_relaxed);
num_.store(0, std::memory_order_relaxed);
sum_.store(0, std::memory_order_relaxed);
sum_squares_.store(0, std::memory_order_relaxed);
for (unsigned int b = 0; b < num_buckets_; b++) {
buckets_[b].store(0, std::memory_order_relaxed);
}
};
bool HistogramStat::Empty() const { return num() == 0; }
void HistogramStat::Add(uint64_t value) {
// This function is designed to be lock free, as it's in the critical path
// of any operation. Each individual value is atomic and the order of updates
// by concurrent threads is tolerable.
const size_t index = bucketMapper.IndexForValue(value);
assert(index < num_buckets_);
buckets_[index].store(buckets_[index].load(std::memory_order_relaxed) + 1,
std::memory_order_relaxed);
uint64_t old_min = min();
if (value < old_min) {
min_.store(value, std::memory_order_relaxed);
}
uint64_t old_max = max();
if (value > old_max) {
max_.store(value, std::memory_order_relaxed);
}
num_.store(num_.load(std::memory_order_relaxed) + 1,
std::memory_order_relaxed);
sum_.store(sum_.load(std::memory_order_relaxed) + value,
std::memory_order_relaxed);
sum_squares_.store(
sum_squares_.load(std::memory_order_relaxed) + value * value,
std::memory_order_relaxed);
}
void HistogramStat::Merge(const HistogramStat& other) {
// This function needs to be performned with the outer lock acquired
// However, atomic operation on every member is still need, since Add()
// requires no lock and value update can still happen concurrently
uint64_t old_min = min();
uint64_t other_min = other.min();
while (other_min < old_min &&
!min_.compare_exchange_weak(old_min, other_min)) {}
uint64_t old_max = max();
uint64_t other_max = other.max();
while (other_max > old_max &&
!max_.compare_exchange_weak(old_max, other_max)) {}
num_.fetch_add(other.num(), std::memory_order_relaxed);
sum_.fetch_add(other.sum(), std::memory_order_relaxed);
sum_squares_.fetch_add(other.sum_squares(), std::memory_order_relaxed);
for (unsigned int b = 0; b < num_buckets_; b++) {
buckets_[b].fetch_add(other.bucket_at(b), std::memory_order_relaxed);
}
}
double HistogramStat::Median() const {
return Percentile(50.0);
}
double HistogramStat::Percentile(double p) const {
double threshold = num() * (p / 100.0);
uint64_t cumulative_sum = 0;
for (unsigned int b = 0; b < num_buckets_; b++) {
uint64_t bucket_value = bucket_at(b);
cumulative_sum += bucket_value;
if (cumulative_sum >= threshold) {
// Scale linearly within this bucket
uint64_t left_point = (b == 0) ? 0 : bucketMapper.BucketLimit(b-1);
uint64_t right_point = bucketMapper.BucketLimit(b);
uint64_t left_sum = cumulative_sum - bucket_value;
uint64_t right_sum = cumulative_sum;
double pos = 0;
uint64_t right_left_diff = right_sum - left_sum;
if (right_left_diff != 0) {
pos = (threshold - left_sum) / right_left_diff;
}
double r = left_point + (right_point - left_point) * pos;
uint64_t cur_min = min();
uint64_t cur_max = max();
if (r < cur_min) r = static_cast<double>(cur_min);
if (r > cur_max) r = static_cast<double>(cur_max);
return r;
}
}
return static_cast<double>(max());
}
double HistogramStat::Average() const {
uint64_t cur_num = num();
uint64_t cur_sum = sum();
if (cur_num == 0) return 0;
return static_cast<double>(cur_sum) / static_cast<double>(cur_num);
}
double HistogramStat::StandardDeviation() const {
uint64_t cur_num = num();
uint64_t cur_sum = sum();
uint64_t cur_sum_squares = sum_squares();
if (cur_num == 0) return 0;
double variance =
static_cast<double>(cur_sum_squares * cur_num - cur_sum * cur_sum) /
static_cast<double>(cur_num * cur_num);
return std::sqrt(variance);
}
std::string HistogramStat::ToString() const {
uint64_t cur_num = num();
std::string r;
char buf[1650];
snprintf(buf, sizeof(buf),
"Count: %" PRIu64 " Average: %.4f StdDev: %.2f\n",
cur_num, Average(), StandardDeviation());
r.append(buf);
snprintf(buf, sizeof(buf),
"Min: %" PRIu64 " Median: %.4f Max: %" PRIu64 "\n",
(cur_num == 0 ? 0 : min()), Median(), (cur_num == 0 ? 0 : max()));
r.append(buf);
snprintf(buf, sizeof(buf),
"Percentiles: "
"P50: %.2f P75: %.2f P99: %.2f P99.9: %.2f P99.99: %.2f\n",
Percentile(50), Percentile(75), Percentile(99), Percentile(99.9),
Percentile(99.99));
r.append(buf);
r.append("------------------------------------------------------\n");
if (cur_num == 0) return r; // all buckets are empty
const double mult = 100.0 / cur_num;
uint64_t cumulative_sum = 0;
for (unsigned int b = 0; b < num_buckets_; b++) {
uint64_t bucket_value = bucket_at(b);
if (bucket_value <= 0.0) continue;
cumulative_sum += bucket_value;
snprintf(buf, sizeof(buf),
"%c %7" PRIu64 ", %7" PRIu64 " ] %8" PRIu64 " %7.3f%% %7.3f%% ",
(b == 0) ? '[' : '(',
(b == 0) ? 0 : bucketMapper.BucketLimit(b-1), // left
bucketMapper.BucketLimit(b), // right
bucket_value, // count
(mult * bucket_value), // percentage
(mult * cumulative_sum)); // cumulative percentage
r.append(buf);
// Add hash marks based on percentage; 20 marks for 100%.
size_t marks = static_cast<size_t>(mult * bucket_value / 5 + 0.5);
r.append(marks, '#');
r.push_back('\n');
}
return r;
}
void HistogramStat::Data(HistogramData * const data) const {
assert(data);
data->median = Median();
data->percentile95 = Percentile(95);
data->percentile99 = Percentile(99);
data->max = static_cast<double>(max());
data->average = Average();
data->standard_deviation = StandardDeviation();
data->count = num();
data->sum = sum();
data->min = static_cast<double>(min());
}
void HistogramImpl::Clear() {
std::lock_guard<std::mutex> lock(mutex_);
stats_.Clear();
}
bool HistogramImpl::Empty() const {
return stats_.Empty();
}
void HistogramImpl::Add(uint64_t value) {
stats_.Add(value);
}
void HistogramImpl::Merge(const Histogram& other) {
if (strcmp(Name(), other.Name()) == 0) {
Merge(
*static_cast_with_check<const HistogramImpl, const Histogram>(&other));
}
}
void HistogramImpl::Merge(const HistogramImpl& other) {
std::lock_guard<std::mutex> lock(mutex_);
stats_.Merge(other.stats_);
}
double HistogramImpl::Median() const {
return stats_.Median();
}
double HistogramImpl::Percentile(double p) const {
return stats_.Percentile(p);
}
double HistogramImpl::Average() const {
return stats_.Average();
}
double HistogramImpl::StandardDeviation() const {
return stats_.StandardDeviation();
}
std::string HistogramImpl::ToString() const {
return stats_.ToString();
}
void HistogramImpl::Data(HistogramData * const data) const {
stats_.Data(data);
}
} // namespace ROCKSDB_NAMESPACE