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181 lines
7.3 KiB
C++
181 lines
7.3 KiB
C++
// -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
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// Copyright (c) 2008, Google Inc.
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above
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// copyright notice, this list of conditions and the following disclaimer
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// in the documentation and/or other materials provided with the
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// distribution.
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// * Neither the name of Google Inc. nor the names of its
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// contributors may be used to endorse or promote products derived from
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// this software without specific prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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// ---
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// All Rights Reserved.
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//
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// Author: Daniel Ford
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#ifndef TCMALLOC_SAMPLER_H_
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#define TCMALLOC_SAMPLER_H_
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#include "config.h"
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#include <stddef.h> // for size_t
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#ifdef HAVE_STDINT_H
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#include <stdint.h> // for uint64_t, uint32_t, int32_t
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#endif
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#include <string.h> // for memcpy
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#include "base/basictypes.h" // for ASSERT
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#include "internal_logging.h" // for ASSERT
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namespace tcmalloc {
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//-------------------------------------------------------------------
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// Sampler to decide when to create a sample trace for an allocation
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// Not thread safe: Each thread should have it's own sampler object.
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// Caller must use external synchronization if used
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// from multiple threads.
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//
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// With 512K average sample step (the default):
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// the probability of sampling a 4K allocation is about 0.00778
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// the probability of sampling a 1MB allocation is about 0.865
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// the probability of sampling a 1GB allocation is about 1.00000
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// In general, the probablity of sampling is an allocation of size X
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// given a flag value of Y (default 1M) is:
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// 1 - e^(-X/Y)
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//
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// With 128K average sample step:
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// the probability of sampling a 1MB allocation is about 0.99966
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// the probability of sampling a 1GB allocation is about 1.0
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// (about 1 - 2**(-26))
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// With 1M average sample step:
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// the probability of sampling a 4K allocation is about 0.00390
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// the probability of sampling a 1MB allocation is about 0.632
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// the probability of sampling a 1GB allocation is about 1.0
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//
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// The sampler works by representing memory as a long stream from
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// which allocations are taken. Some of the bytes in this stream are
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// marked and if an allocation includes a marked byte then it is
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// sampled. Bytes are marked according to a Poisson point process
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// with each byte being marked independently with probability
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// p = 1/tcmalloc_sample_parameter. This makes the probability
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// of sampling an allocation of X bytes equal to the CDF of
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// a geometric with mean tcmalloc_sample_parameter. (ie. the
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// probability that at least one byte in the range is marked). This
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// is accurately given by the CDF of the corresponding exponential
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// distribution : 1 - e^(X/tcmalloc_sample_parameter_)
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// Independence of the byte marking ensures independence of
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// the sampling of each allocation.
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//
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// This scheme is implemented by noting that, starting from any
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// fixed place, the number of bytes until the next marked byte
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// is geometrically distributed. This number is recorded as
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// bytes_until_sample_. Every allocation subtracts from this
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// number until it is less than 0. When this happens the current
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// allocation is sampled.
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//
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// When an allocation occurs, bytes_until_sample_ is reset to
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// a new independtly sampled geometric number of bytes. The
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// memoryless property of the point process means that this may
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// be taken as the number of bytes after the end of the current
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// allocation until the next marked byte. This ensures that
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// very large allocations which would intersect many marked bytes
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// only result in a single call to PickNextSamplingPoint.
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//-------------------------------------------------------------------
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class PERFTOOLS_DLL_DECL Sampler {
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public:
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// Initialize this sampler.
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// Passing a seed of 0 gives a non-deterministic
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// seed value given by casting the object ("this")
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void Init(uint32_t seed);
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void Cleanup();
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// Record allocation of "k" bytes. Return true iff allocation
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// should be sampled
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bool SampleAllocation(size_t k);
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// Generate a geometric with mean 512K (or FLAG_tcmalloc_sample_parameter)
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size_t PickNextSamplingPoint();
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// Initialize the statics for the Sampler class
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static void InitStatics();
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// Returns the current sample period
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int GetSamplePeriod();
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// The following are public for the purposes of testing
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static uint64_t NextRandom(uint64_t rnd_); // Returns the next prng value
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static double FastLog2(const double & d); // Computes Log2(x) quickly
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static void PopulateFastLog2Table(); // Populate the lookup table
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private:
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size_t bytes_until_sample_; // Bytes until we sample next
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uint64_t rnd_; // Cheap random number generator
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// Statics for the fast log
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// Note that this code may not depend on anything in //util
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// hence the duplication of functionality here
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static const int kFastlogNumBits = 10;
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static const int kFastlogMask = (1 << kFastlogNumBits) - 1;
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static double log_table_[1<<kFastlogNumBits]; // Constant
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};
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inline bool Sampler::SampleAllocation(size_t k) {
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if (bytes_until_sample_ < k) {
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bytes_until_sample_ = PickNextSamplingPoint();
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return true;
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} else {
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bytes_until_sample_ -= k;
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return false;
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}
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}
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// Inline functions which are public for testing purposes
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// Returns the next prng value.
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// pRNG is: aX+b mod c with a = 0x5DEECE66D, b = 0xB, c = 1<<48
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// This is the lrand64 generator.
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inline uint64_t Sampler::NextRandom(uint64_t rnd) {
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const uint64_t prng_mult = 0x5DEECE66DLL;
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const uint64_t prng_add = 0xB;
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const uint64_t prng_mod_power = 48;
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const uint64_t prng_mod_mask =
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~((~static_cast<uint64_t>(0)) << prng_mod_power);
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return (prng_mult * rnd + prng_add) & prng_mod_mask;
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}
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// Adapted from //util/math/fastmath.[h|cc] by Noam Shazeer
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// This mimics the VeryFastLog2 code in those files
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inline double Sampler::FastLog2(const double & d) {
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ASSERT(d>0);
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COMPILE_ASSERT(sizeof(d) == sizeof(uint64_t), DoubleMustBe64Bits);
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uint64_t x;
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memcpy(&x, &d, sizeof(x)); // we depend on the compiler inlining this
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const uint32_t x_high = x >> 32;
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const uint32_t y = x_high >> (20 - kFastlogNumBits) & kFastlogMask;
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const int32_t exponent = ((x_high >> 20) & 0x7FF) - 1023;
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return exponent + log_table_[y];
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}
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} // namespace tcmalloc
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#endif // TCMALLOC_SAMPLER_H_
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