src/sampler.cc

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// ---
// All Rights Reserved.
//
// Author: Daniel Ford

#include "sampler.h"

#include <algorithm>  // For min()
#include <math.h>

#include "base/commandlineflags.h"
#include "base/spinlock.h"
#include "base/sysinfo.h"

// The approximate gap in bytes between sampling actions.  See Init
// below for how it is initialized from TCMALLOC_SAMPLE_PARAMETER
// environment variable.
//
// 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.
DEFINE_int64(tcmalloc_sample_parameter, 0, "");

namespace tcmalloc {

int Sampler::GetSamplePeriod() {
  return FLAGS_tcmalloc_sample_parameter;
}

// Run this before using your sampler
void Sampler::Init(uint64_t seed) {
  DCHECK_NE(seed, 0);

  // Initialize PRNG
  rnd_ = seed;
  // Step it forward 20 times for good measure
  for (int i = 0; i < 20; i++) {
    rnd_ = NextRandom(rnd_);
  }

#ifndef NO_TCMALLOC_SAMPLES
  static TrivialOnce setup_parameter;
  setup_parameter.RunOnce([] () {
    const char* val = GetenvBeforeMain("TCMALLOC_SAMPLE_PARAMETER");
    FLAGS_tcmalloc_sample_parameter = tcmalloc::commandlineflags::StringToLongLong(val, 0);
  });
#endif

  // Initialize counter
  bytes_until_sample_ = PickNextSamplingPoint();
}

static constexpr auto kMaxSSize = (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() {
  if (FLAGS_tcmalloc_sample_parameter <= 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) * FLAGS_tcmalloc_sample_parameter);

  // 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<double>(interval, static_cast<double>(kMaxSSize)));
}

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 FLAGS_tcmalloc_sample_parameter <= 0;
}

}  // namespace tcmalloc