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// -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
// Copyright (c) 2008, Google Inc.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
// ---
// All Rights Reserved.
//
// Author: Daniel Ford
#include "sampler.h"
#include <algorithm> // For min()
#include <math.h>
#include "base/commandlineflags.h"
using std::min;
// The approximate gap in bytes between sampling actions.
// I.e., we take one sample approximately once every
// tcmalloc_sample_parameter bytes of allocation
// i.e. about once every 512KB if value is 1<<19.
#ifdef NO_TCMALLOC_SAMPLES
DEFINE_int64(tcmalloc_sample_parameter, 0,
"Unused: code is compiled with NO_TCMALLOC_SAMPLES");
#else
DEFINE_int64(tcmalloc_sample_parameter,
EnvToInt64("TCMALLOC_SAMPLE_PARAMETER", 0),
"The approximate gap in bytes between sampling actions. "
"This must be between 1 and 2^58.");
#endif
namespace tcmalloc {
// Statics for Sampler
double Sampler::log_table_[1<<kFastlogNumBits];
// Populate the lookup table for FastLog2.
// This approximates the log2 curve with a step function.
// Steps have height equal to log2 of the mid-point of the step.
void Sampler::PopulateFastLog2Table() {
for (int i = 0; i < (1<<kFastlogNumBits); i++) {
log_table_[i] = (log(1.0 + static_cast<double>(i+0.5)/(1<<kFastlogNumBits))
/ log(2.0));
}
}
int Sampler::GetSamplePeriod() {
return FLAGS_tcmalloc_sample_parameter;
}
// Run this before using your sampler
void Sampler::Init(uint32_t seed) {
// Initialize PRNG
if (seed != 0) {
rnd_ = seed;
} else {
rnd_ = static_cast<uint32_t>(reinterpret_cast<uintptr_t>(this));
if (rnd_ == 0) {
rnd_ = 1;
}
}
// Step it forward 20 times for good measure
for (int i = 0; i < 20; i++) {
rnd_ = NextRandom(rnd_);
}
// Initialize counter
bytes_until_sample_ = PickNextSamplingPoint();
}
// Initialize the Statics for the Sampler class
void Sampler::InitStatics() {
PopulateFastLog2Table();
}
// Generates a geometric variable with the specified mean (512K by default).
// This is done by generating a random number between 0 and 1 and applying
// the inverse cumulative distribution function for an exponential.
// Specifically: Let m be the inverse of the sample period, then
// the probability distribution function is m*exp(-mx) so the CDF is
// p = 1 - exp(-mx), so
// q = 1 - p = exp(-mx)
// log_e(q) = -mx
// -log_e(q)/m = x
// log_2(q) * (-log_e(2) * 1/m) = x
// In the code, q is actually in the range 1 to 2**26, hence the -26 below
size_t Sampler::PickNextSamplingPoint() {
rnd_ = NextRandom(rnd_);
// Take the top 26 bits as the random number
// (This plus the 1<<58 sampling bound give a max possible step of
// 5194297183973780480 bytes.)
const uint64_t prng_mod_power = 48; // Number of bits in prng
// The uint32_t cast is to prevent a (hard-to-reproduce) NAN
// under piii debug for some binaries.
double q = static_cast<uint32_t>(rnd_ >> (prng_mod_power - 26)) + 1.0;
// Put the computed p-value through the CDF of a geometric.
// For faster performance (save ~1/20th exec time), replace
// min(0.0, FastLog2(q) - 26) by (Fastlog2(q) - 26.000705)
// The value 26.000705 is used rather than 26 to compensate
// for inaccuracies in FastLog2 which otherwise result in a
// negative answer.
return static_cast<size_t>(min(0.0, (FastLog2(q) - 26)) * (-log(2.0)
* FLAGS_tcmalloc_sample_parameter) + 1);
}
} // namespace tcmalloc
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