third_party/tcmalloc/vendor/src/sampler.cc

   1 // Copyright (c) 2008, Google Inc.
   2 // All rights reserved.
   3 //
   4 // Redistribution and use in source and binary forms, with or without
   5 // modification, are permitted provided that the following conditions are
   6 // met:
   7 //
   8 //     * Redistributions of source code must retain the above copyright
   9 // notice, this list of conditions and the following disclaimer.
  10 //     * Redistributions in binary form must reproduce the above
  11 // copyright notice, this list of conditions and the following disclaimer
  12 // in the documentation and/or other materials provided with the
  13 // distribution.
  14 //     * Neither the name of Google Inc. nor the names of its
  15 // contributors may be used to endorse or promote products derived from
  16 // this software without specific prior written permission.
  17 //
  18 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  19 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  20 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  21 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  22 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  23 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  24 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  25 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  26 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  27 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  28 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  29
  30 // ---
  31 // All Rights Reserved.
  32 //
  33 // Author: Daniel Ford
  34
  35 #include "sampler.h"
  36
  37 #include <algorithm>  // For min()
  38 #include <math.h>
  39 #include "base/commandlineflags.h"
  40
  41 using std::min;
  42
  43 // The approximate gap in bytes between sampling actions.
  44 // I.e., we take one sample approximately once every
  45 // tcmalloc_sample_parameter bytes of allocation
  46 // i.e. about once every 512KB if value is 1<<19.
  47 #ifdef NO_TCMALLOC_SAMPLES
  48 DEFINE_int64(tcmalloc_sample_parameter, 0,
  49              "Unused: code is compiled with NO_TCMALLOC_SAMPLES");
  50 #else
  51 DEFINE_int64(tcmalloc_sample_parameter,
  52              EnvToInt64("TCMALLOC_SAMPLE_PARAMETER", 0),
  53              "The approximate gap in bytes between sampling actions. "
  54              "This must be between 1 and 2^58.");
  55 #endif
  56
  57 namespace tcmalloc {
  58
  59 // Statics for Sampler
  60 double Sampler::log_table_[1<<kFastlogNumBits];
  61
  62 // Populate the lookup table for FastLog2.
  63 // This approximates the log2 curve with a step function.
  64 // Steps have height equal to log2 of the mid-point of the step.
  65 void Sampler::PopulateFastLog2Table() {
  66   for (int i = 0; i < (1<<kFastlogNumBits); i++) {
  67     log_table_[i] = (log(1.0 + static_cast<double>(i+0.5)/(1<<kFastlogNumBits))
  68                      / log(2.0));
  69   }
  70 }
  71
  72 int Sampler::GetSamplePeriod() {
  73   return FLAGS_tcmalloc_sample_parameter;
  74 }
  75
  76 // Run this before using your sampler
  77 void Sampler::Init(uint32_t seed) {
  78   // Initialize PRNG
  79   if (seed != 0) {
  80     rnd_ = seed;
  81   } else {
  82     rnd_ = static_cast<uint32_t>(reinterpret_cast<uintptr_t>(this));
  83     if (rnd_ == 0) {
  84       rnd_ = 1;
  85     }
  86   }
  87   // Step it forward 20 times for good measure
  88   for (int i = 0; i < 20; i++) {
  89     rnd_ = NextRandom(rnd_);
  90   }
  91   // Initialize counter
  92   bytes_until_sample_ = PickNextSamplingPoint();
  93 }
  94
  95 // Initialize the Statics for the Sampler class
  96 void Sampler::InitStatics() {
  97   PopulateFastLog2Table();
  98 }
  99
 100 // Generates a geometric variable with the specified mean (512K by default).
 101 // This is done by generating a random number between 0 and 1 and applying
 102 // the inverse cumulative distribution function for an exponential.
 103 // Specifically: Let m be the inverse of the sample period, then
 104 // the probability distribution function is m*exp(-mx) so the CDF is
 105 // p = 1 - exp(-mx), so
 106 // q = 1 - p = exp(-mx)
 107 // log_e(q) = -mx
 108 // -log_e(q)/m = x
 109 // log_2(q) * (-log_e(2) * 1/m) = x
 110 // In the code, q is actually in the range 1 to 2**26, hence the -26 below
 111 size_t Sampler::PickNextSamplingPoint() {
 112   rnd_ = NextRandom(rnd_);
 113   // Take the top 26 bits as the random number
 114   // (This plus the 1<<58 sampling bound give a max possible step of
 115   // 5194297183973780480 bytes.)
 116   const uint64_t prng_mod_power = 48;  // Number of bits in prng
 117   // The uint32_t cast is to prevent a (hard-to-reproduce) NAN
 118   // under piii debug for some binaries.
 119   double q = static_cast<uint32_t>(rnd_ >> (prng_mod_power - 26)) + 1.0;
 120   // Put the computed p-value through the CDF of a geometric.
 121   // For faster performance (save ~1/20th exec time), replace
 122   // min(0.0, FastLog2(q) - 26)  by  (Fastlog2(q) - 26.000705)
 123   // The value 26.000705 is used rather than 26 to compensate
 124   // for inaccuracies in FastLog2 which otherwise result in a
 125   // negative answer.
 126   return static_cast<size_t>(min(0.0, (FastLog2(q) - 26)) * (-log(2.0)
 127                              * FLAGS_tcmalloc_sample_parameter) + 1);
 128 }
 129
 130 }  // namespace tcmalloc