LevelDB/util/histogram.cc

// 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 "util/histogram.h"

#include <cmath>
#include <cstdio>

#include "port/port.h"

namespace leveldb {

const double Histogram::kBucketLimit[kNumBuckets] = {
    1,
    2,
    3,
    4,
    5,
    6,
    7,
    8,
    9,
    10,
    12,
    14,
    16,
    18,
    20,
    25,
    30,
    35,
    40,
    45,
    50,
    60,
    70,
    80,
    90,
    100,
    120,
    140,
    160,
    180,
    200,
    250,
    300,
    350,
    400,
    450,
    500,
    600,
    700,
    800,
    900,
    1000,
    1200,
    1400,
    1600,
    1800,
    2000,
    2500,
    3000,
    3500,
    4000,
    4500,
    5000,
    6000,
    7000,
    8000,
    9000,
    10000,
    12000,
    14000,
    16000,
    18000,
    20000,
    25000,
    30000,
    35000,
    40000,
    45000,
    50000,
    60000,
    70000,
    80000,
    90000,
    100000,
    120000,
    140000,
    160000,
    180000,
    200000,
    250000,
    300000,
    350000,
    400000,
    450000,
    500000,
    600000,
    700000,
    800000,
    900000,
    1000000,
    1200000,
    1400000,
    1600000,
    1800000,
    2000000,
    2500000,
    3000000,
    3500000,
    4000000,
    4500000,
    5000000,
    6000000,
    7000000,
    8000000,
    9000000,
    10000000,
    12000000,
    14000000,
    16000000,
    18000000,
    20000000,
    25000000,
    30000000,
    35000000,
    40000000,
    45000000,
    50000000,
    60000000,
    70000000,
    80000000,
    90000000,
    100000000,
    120000000,
    140000000,
    160000000,
    180000000,
    200000000,
    250000000,
    300000000,
    350000000,
    400000000,
    450000000,
    500000000,
    600000000,
    700000000,
    800000000,
    900000000,
    1000000000,
    1200000000,
    1400000000,
    1600000000,
    1800000000,
    2000000000,
    2500000000.0,
    3000000000.0,
    3500000000.0,
    4000000000.0,
    4500000000.0,
    5000000000.0,
    6000000000.0,
    7000000000.0,
    8000000000.0,
    9000000000.0,
    1e200,
};

void Histogram::Clear() {
  min_ = kBucketLimit[kNumBuckets - 1];
  max_ = 0;
  num_ = 0;
  sum_ = 0;
  sum_squares_ = 0;
  for (int i = 0; i < kNumBuckets; i++) {
    buckets_[i] = 0;
  }
}

void Histogram::Add(double value) {
  // Linear search is fast enough for our usage in db_bench
  int b = 0;
  while (b < kNumBuckets - 1 && kBucketLimit[b] <= value) {
    b++;
  }
  buckets_[b] += 1.0;
  if (min_ > value) min_ = value;
  if (max_ < value) max_ = value;
  num_++;
  sum_ += value;
  sum_squares_ += (value * value);
}

void Histogram::Merge(const Histogram& other) {
  if (other.min_ < min_) min_ = other.min_;
  if (other.max_ > max_) max_ = other.max_;
  num_ += other.num_;
  sum_ += other.sum_;
  sum_squares_ += other.sum_squares_;
  for (int b = 0; b < kNumBuckets; b++) {
    buckets_[b] += other.buckets_[b];
  }
}

double Histogram::Median() const { return Percentile(50.0); }

double Histogram::Percentile(double p) const {
  double threshold = num_ * (p / 100.0);
  double sum = 0;
  for (int b = 0; b < kNumBuckets; b++) {
    sum += buckets_[b];
    if (sum >= threshold) {
      // Scale linearly within this bucket
      double left_point = (b == 0) ? 0 : kBucketLimit[b - 1];
      double right_point = kBucketLimit[b];
      double left_sum = sum - buckets_[b];
      double right_sum = sum;
      double pos = (threshold - left_sum) / (right_sum - left_sum);
      double r = left_point + (right_point - left_point) * pos;
      if (r < min_) r = min_;
      if (r > max_) r = max_;
      return r;
    }
  }
  return max_;
}

double Histogram::Average() const {
  if (num_ == 0.0) return 0;
  return sum_ / num_;
}

double Histogram::StandardDeviation() const {
  if (num_ == 0.0) return 0;
  double variance = (sum_squares_ * num_ - sum_ * sum_) / (num_ * num_);
  return sqrt(variance);
}

std::string Histogram::ToString() const {
  std::string r;
  char buf[200];
  std::snprintf(buf, sizeof(buf), "Count: %.0f  Average: %.4f  StdDev: %.2f\n",
                num_, Average(), StandardDeviation());
  r.append(buf);
  std::snprintf(buf, sizeof(buf), "Min: %.4f  Median: %.4f  Max: %.4f\n",
                (num_ == 0.0 ? 0.0 : min_), Median(), max_);
  r.append(buf);
  r.append("------------------------------------------------------\n");
  const double mult = 100.0 / num_;
  double sum = 0;
  for (int b = 0; b < kNumBuckets; b++) {
    if (buckets_[b] <= 0.0) continue;
    sum += buckets_[b];
    std::snprintf(buf, sizeof(buf), "[ %7.0f, %7.0f ) %7.0f %7.3f%% %7.3f%% ",
                  ((b == 0) ? 0.0 : kBucketLimit[b - 1]),  // left
                  kBucketLimit[b],                         // right
                  buckets_[b],                             // count
                  mult * buckets_[b],                      // percentage
                  mult * sum);  // cumulative percentage
    r.append(buf);

    // Add hash marks based on percentage; 20 marks for 100%.
    int marks = static_cast<int>(20 * (buckets_[b] / num_) + 0.5);
    r.append(marks, '#');
    r.push_back('\n');
  }
  return r;
}

}  // namespace leveldb