third_party/tcmalloc/chromium/src/sampler.h

   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 #ifndef TCMALLOC_SAMPLER_H_
  36 #define TCMALLOC_SAMPLER_H_
  37
  38 #include "config.h"
  39 #include <stddef.h>                     // for size_t
  40 #ifdef HAVE_STDINT_H
  41 #include <stdint.h>                     // for uint64_t, uint32_t, int32_t
  42 #endif
  43 #include <string.h>                     // for memcpy
  44 #include "base/basictypes.h"  // for ASSERT
  45 #include "internal_logging.h"  // for ASSERT
  46
  47 namespace tcmalloc {
  48
  49 //-------------------------------------------------------------------
  50 // Sampler to decide when to create a sample trace for an allocation
  51 // Not thread safe: Each thread should have it's own sampler object.
  52 // Caller must use external synchronization if used
  53 // from multiple threads.
  54 //
  55 // With 512K average sample step (the default):
  56 //  the probability of sampling a 4K allocation is about 0.00778
  57 //  the probability of sampling a 1MB allocation is about 0.865
  58 //  the probability of sampling a 1GB allocation is about 1.00000
  59 // In general, the probablity of sampling is an allocation of size X
  60 // given a flag value of Y (default 1M) is:
  61 //  1 - e^(-X/Y)
  62 //
  63 // With 128K average sample step:
  64 //  the probability of sampling a 1MB allocation is about 0.99966
  65 //  the probability of sampling a 1GB allocation is about 1.0
  66 //  (about 1 - 2**(-26))
  67 // With 1M average sample step:
  68 //  the probability of sampling a 4K allocation is about 0.00390
  69 //  the probability of sampling a 1MB allocation is about 0.632
  70 //  the probability of sampling a 1GB allocation is about 1.0
  71 //
  72 // The sampler works by representing memory as a long stream from
  73 // which allocations are taken. Some of the bytes in this stream are
  74 // marked and if an allocation includes a marked byte then it is
  75 // sampled. Bytes are marked according to a Poisson point process
  76 // with each byte being marked independently with probability
  77 // p = 1/tcmalloc_sample_parameter.  This makes the probability
  78 // of sampling an allocation of X bytes equal to the CDF of
  79 // a geometric with mean tcmalloc_sample_parameter. (ie. the
  80 // probability that at least one byte in the range is marked). This
  81 // is accurately given by the CDF of the corresponding exponential
  82 // distribution : 1 - e^(X/tcmalloc_sample_parameter_)
  83 // Independence of the byte marking ensures independence of
  84 // the sampling of each allocation.
  85 //
  86 // This scheme is implemented by noting that, starting from any
  87 // fixed place, the number of bytes until the next marked byte
  88 // is geometrically distributed. This number is recorded as
  89 // bytes_until_sample_.  Every allocation subtracts from this
  90 // number until it is less than 0. When this happens the current
  91 // allocation is sampled.
  92 //
  93 // When an allocation occurs, bytes_until_sample_ is reset to
  94 // a new independtly sampled geometric number of bytes. The
  95 // memoryless property of the point process means that this may
  96 // be taken as the number of bytes after the end of the current
  97 // allocation until the next marked byte. This ensures that
  98 // very large allocations which would intersect many marked bytes
  99 // only result in a single call to PickNextSamplingPoint.
 100 //-------------------------------------------------------------------
 101
 102 class PERFTOOLS_DLL_DECL Sampler {
 103  public:
 104   // Initialize this sampler.
 105   // Passing a seed of 0 gives a non-deterministic
 106   // seed value given by casting the object ("this")
 107   void Init(uint32_t seed);
 108   void Cleanup();
 109
 110   // Record allocation of "k" bytes.  Return true iff allocation
 111   // should be sampled
 112   bool SampleAllocation(size_t k);
 113
 114   // Generate a geometric with mean 512K (or FLAG_tcmalloc_sample_parameter)
 115   size_t PickNextSamplingPoint();
 116
 117   // Initialize the statics for the Sampler class
 118   static void InitStatics();
 119
 120   // Returns the current sample period
 121   int GetSamplePeriod();
 122
 123   // The following are public for the purposes of testing
 124   static uint64_t NextRandom(uint64_t rnd_);  // Returns the next prng value
 125   static double FastLog2(const double & d);  // Computes Log2(x) quickly
 126   static void PopulateFastLog2Table();  // Populate the lookup table
 127
 128  private:
 129   size_t        bytes_until_sample_;    // Bytes until we sample next
 130   uint64_t      rnd_;                   // Cheap random number generator
 131
 132   // Statics for the fast log
 133   // Note that this code may not depend on anything in //util
 134   // hence the duplication of functionality here
 135   static const int kFastlogNumBits = 10;
 136   static const int kFastlogMask = (1 << kFastlogNumBits) - 1;
 137   static double log_table_[1<<kFastlogNumBits];  // Constant
 138 };
 139
 140 inline bool Sampler::SampleAllocation(size_t k) {
 141   if (bytes_until_sample_ < k) {
 142     bytes_until_sample_ = PickNextSamplingPoint();
 143     return true;
 144   } else {
 145     bytes_until_sample_ -= k;
 146     return false;
 147   }
 148 }
 149
 150 // Inline functions which are public for testing purposes
 151
 152 // Returns the next prng value.
 153 // pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48
 154 // This is the lrand64 generator.
 155 inline uint64_t Sampler::NextRandom(uint64_t rnd) {
 156   const uint64_t prng_mult = 0x5DEECE66DLL;
 157   const uint64_t prng_add = 0xB;
 158   const uint64_t prng_mod_power = 48;
 159   const uint64_t prng_mod_mask =
 160                 ~((~static_cast<uint64_t>(0)) << prng_mod_power);
 161   return (prng_mult * rnd + prng_add) & prng_mod_mask;
 162 }
 163
 164 // Adapted from //util/math/fastmath.[h|cc] by Noam Shazeer
 165 // This mimics the VeryFastLog2 code in those files
 166 inline double Sampler::FastLog2(const double & d) {
 167   ASSERT(d>0);
 168   COMPILE_ASSERT(sizeof(d) == sizeof(uint64_t), DoubleMustBe64Bits);
 169   uint64_t x;
 170   memcpy(&x, &d, sizeof(x));   // we depend on the compiler inlining this
 171   const uint32_t x_high = x >> 32;
 172   const uint32_t y = x_high >> (20 - kFastlogNumBits) & kFastlogMask;
 173   const int32_t exponent = ((x_high >> 20) & 0x7FF) - 1023;
 174   return exponent + log_table_[y];
 175 }
 176
 177 }  // namespace tcmalloc
 178
 179 #endif  // TCMALLOC_SAMPLER_H_