// Copyright (c) 2008, Google Inc.
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//
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// All Rights Reserved.
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// 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_