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 //
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9 // notice, this list of conditions and the following disclaimer.
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11 // copyright notice, this list of conditions and the following disclaimer
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13 // distribution.
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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
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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
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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.
PopulateFastLog2Table()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
GetSamplePeriod()72 int Sampler::GetSamplePeriod() {
73 return FLAGS_tcmalloc_sample_parameter;
74 }
75
76 // Run this before using your sampler
Init(uint32_t seed)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
InitStatics()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
PickNextSamplingPoint()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