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 41using 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 48DEFINE_int64(tcmalloc_sample_parameter, 0, 49 "Unused: code is compiled with NO_TCMALLOC_SAMPLES"); 50#else 51DEFINE_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 57namespace tcmalloc { 58 59// Statics for Sampler 60double 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. 65void 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 72int Sampler::GetSamplePeriod() { 73 return FLAGS_tcmalloc_sample_parameter; 74} 75 76// Run this before using your sampler 77void 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 96void 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 111size_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 131