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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 <math.h>
#include <algorithm> // For min()
using std::min;
namespace tcmalloc {
// 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
std::atomic<size_t> Sampler::tcmalloc_sample_parameter_(0);
#else
std::atomic<size_t> Sampler::tcmalloc_sample_parameter_(
EnvToInt64("TCMALLOC_SAMPLE_PARAMETER", 0));
#endif
// Static
size_t Sampler::GetSamplePeriod() {
return tcmalloc_sample_parameter_.load();
}
// Static
void Sampler::SetSamplePeriod(size_t period) {
tcmalloc_sample_parameter_.store(period);
}
// Run this before using your sampler
void Sampler::Init(uint64_t seed) {
ASSERT(seed != 0);
// Initialize PRNG
rnd_ = seed;
// Step it forward 20 times for good measure
for (int i = 0; i < 20; i++) {
rnd_ = NextRandom(rnd_);
}
// Initialize counter
bytes_until_sample_ = PickNextSamplingPoint();
}
#define MAX_SSIZE (static_cast<ssize_t>(static_cast<size_t>(static_cast<ssize_t>(-1)) >> 1))
// 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
ssize_t Sampler::PickNextSamplingPoint() {
size_t sampling_period = GetSamplePeriod();
if (sampling_period == 0) {
// In this case, we don't want to sample ever, and the larger a
// value we put here, the longer until we hit the slow path
// again. However, we have to support the flag changing at
// runtime, so pick something reasonably large (to keep overhead
// low) but small enough that we'll eventually start to sample
// again.
return 16 << 20;
}
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.
double interval = (log2(q) - 26) * (-log(2.0) * sampling_period);
// Very large values of interval overflow ssize_t. If we happen to
// hit such improbable condition, we simply cheat and clamp interval
// to largest supported value.
return static_cast<ssize_t>(
std::min(interval, static_cast<double>(MAX_SSIZE)));
}
bool Sampler::RecordAllocationSlow(size_t k) {
if (!initialized_) {
initialized_ = true;
Init(reinterpret_cast<uintptr_t>(this));
if (static_cast<size_t>(bytes_until_sample_) >= k) {
bytes_until_sample_ -= k;
return true;
}
}
bytes_until_sample_ = PickNextSamplingPoint();
return GetSamplePeriod() == 0;
}
} // namespace tcmalloc