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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
- #ifndef TCMALLOC_SAMPLER_H_
- #define TCMALLOC_SAMPLER_H_
- #include "config.h"
- #include <stddef.h> // for size_t
- #ifdef HAVE_STDINT_H
- #include <stdint.h> // for uint64_t, uint32_t, int32_t
- #endif
- #include <string.h> // for memcpy
- #include "base/basictypes.h" // for ASSERT
- #include "internal_logging.h" // for ASSERT
- namespace tcmalloc {
- //-------------------------------------------------------------------
- // Sampler to decide when to create a sample trace for an allocation
- // Not thread safe: Each thread should have it's own sampler object.
- // Caller must use external synchronization if used
- // from multiple threads.
- //
- // With 512K average sample step (the default):
- // the probability of sampling a 4K allocation is about 0.00778
- // the probability of sampling a 1MB allocation is about 0.865
- // the probability of sampling a 1GB allocation is about 1.00000
- // In general, the probablity of sampling is an allocation of size X
- // given a flag value of Y (default 1M) is:
- // 1 - e^(-X/Y)
- //
- // With 128K average sample step:
- // the probability of sampling a 1MB allocation is about 0.99966
- // the probability of sampling a 1GB allocation is about 1.0
- // (about 1 - 2**(-26))
- // With 1M average sample step:
- // the probability of sampling a 4K allocation is about 0.00390
- // the probability of sampling a 1MB allocation is about 0.632
- // the probability of sampling a 1GB allocation is about 1.0
- //
- // The sampler works by representing memory as a long stream from
- // which allocations are taken. Some of the bytes in this stream are
- // marked and if an allocation includes a marked byte then it is
- // sampled. Bytes are marked according to a Poisson point process
- // with each byte being marked independently with probability
- // p = 1/tcmalloc_sample_parameter. This makes the probability
- // of sampling an allocation of X bytes equal to the CDF of
- // a geometric with mean tcmalloc_sample_parameter. (ie. the
- // probability that at least one byte in the range is marked). This
- // is accurately given by the CDF of the corresponding exponential
- // distribution : 1 - e^(X/tcmalloc_sample_parameter_)
- // Independence of the byte marking ensures independence of
- // the sampling of each allocation.
- //
- // This scheme is implemented by noting that, starting from any
- // fixed place, the number of bytes until the next marked byte
- // is geometrically distributed. This number is recorded as
- // bytes_until_sample_. Every allocation subtracts from this
- // number until it is less than 0. When this happens the current
- // allocation is sampled.
- //
- // When an allocation occurs, bytes_until_sample_ is reset to
- // a new independtly sampled geometric number of bytes. The
- // memoryless property of the point process means that this may
- // be taken as the number of bytes after the end of the current
- // allocation until the next marked byte. This ensures that
- // very large allocations which would intersect many marked bytes
- // only result in a single call to PickNextSamplingPoint.
- //-------------------------------------------------------------------
- class PERFTOOLS_DLL_DECL Sampler {
- public:
- // Initialize this sampler.
- // Passing a seed of 0 gives a non-deterministic
- // seed value given by casting the object ("this")
- void Init(uint32_t seed);
- void Cleanup();
- // Record allocation of "k" bytes. Return true iff allocation
- // should be sampled
- bool SampleAllocation(size_t k);
- // Generate a geometric with mean 512K (or FLAG_tcmalloc_sample_parameter)
- size_t PickNextSamplingPoint();
- // Initialize the statics for the Sampler class
- static void InitStatics();
- // Returns the current sample period
- int GetSamplePeriod();
- // The following are public for the purposes of testing
- static uint64_t NextRandom(uint64_t rnd_); // Returns the next prng value
- static double FastLog2(const double & d); // Computes Log2(x) quickly
- static void PopulateFastLog2Table(); // Populate the lookup table
- private:
- size_t bytes_until_sample_; // Bytes until we sample next
- uint64_t rnd_; // Cheap random number generator
- // Statics for the fast log
- // Note that this code may not depend on anything in //util
- // hence the duplication of functionality here
- static const int kFastlogNumBits = 10;
- static const int kFastlogMask = (1 << kFastlogNumBits) - 1;
- static double log_table_[1<<kFastlogNumBits]; // Constant
- };
- inline bool Sampler::SampleAllocation(size_t k) {
- if (bytes_until_sample_ < k) {
- bytes_until_sample_ = PickNextSamplingPoint();
- return true;
- } else {
- bytes_until_sample_ -= k;
- return false;
- }
- }
- // Inline functions which are public for testing purposes
- // Returns the next prng value.
- // pRNG is: aX+b mod c with a = 0x5DEECE66D, b = 0xB, c = 1<<48
- // This is the lrand64 generator.
- inline uint64_t Sampler::NextRandom(uint64_t rnd) {
- const uint64_t prng_mult = 0x5DEECE66DLL;
- const uint64_t prng_add = 0xB;
- const uint64_t prng_mod_power = 48;
- const uint64_t prng_mod_mask =
- ~((~static_cast<uint64_t>(0)) << prng_mod_power);
- return (prng_mult * rnd + prng_add) & prng_mod_mask;
- }
- // Adapted from //util/math/fastmath.[h|cc] by Noam Shazeer
- // This mimics the VeryFastLog2 code in those files
- inline double Sampler::FastLog2(const double & d) {
- ASSERT(d>0);
- COMPILE_ASSERT(sizeof(d) == sizeof(uint64_t), DoubleMustBe64Bits);
- uint64_t x;
- memcpy(&x, &d, sizeof(x)); // we depend on the compiler inlining this
- const uint32_t x_high = x >> 32;
- const uint32_t y = x_high >> (20 - kFastlogNumBits) & kFastlogMask;
- const int32_t exponent = ((x_high >> 20) & 0x7FF) - 1023;
- return exponent + log_table_[y];
- }
- } // namespace tcmalloc
- #endif // TCMALLOC_SAMPLER_H_
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