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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.
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