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 131
