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BeefRT/gperftools/src/sampler.cc

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+// 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;
+
+namespace TCMALLOC_NAMESPACE
+{
+	// 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_NAMESPACE {
+
+// 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_NAMESPACE