about summary refs log tree commit diff
path: root/absl/base/internal/periodic_sampler.h
blob: 2c0600f05ea4177c455aa93bc421a6aa8fbb2fe4 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
#define ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_

#include <stdint.h>

#include <atomic>

#include "absl/base/internal/exponential_biased.h"
#include "absl/base/optimization.h"

namespace absl {
namespace base_internal {

// PeriodicSamplerBase provides the basic period sampler implementation.
//
// This is the base class for the templated PeriodicSampler class, which holds
// a global std::atomic value identified by a user defined tag, such that
// each specific PeriodSampler implementation holds its own global period.
//
// PeriodicSamplerBase is thread-compatible except where stated otherwise.
class PeriodicSamplerBase {
 public:
  // PeriodicSamplerBase is trivial / copyable / movable / destructible.
  PeriodicSamplerBase() = default;
  PeriodicSamplerBase(PeriodicSamplerBase&&) = default;
  PeriodicSamplerBase(const PeriodicSamplerBase&) = default;

  // Returns true roughly once every `period` calls. This is established by a
  // randomly picked `stride` that is counted down on each call to `Sample`.
  // This stride is picked such that the probability of `Sample()` returning
  // true is 1 in `period`.
  inline bool Sample() noexcept;

  // The below methods are intended for optimized use cases where the
  // size of the inlined fast path code is highly important. Applications
  // should use the `Sample()` method unless they have proof that their
  // specific use case requires the optimizations offered by these methods.
  //
  // An example of such a use case is SwissTable sampling. All sampling checks
  // are in inlined SwissTable methods, and the number of call sites is huge.
  // In this case, the inlined code size added to each translation unit calling
  // SwissTable methods is non-trivial.
  //
  // The `SubtleMaybeSample()` function spuriously returns true even if the
  // function should not be sampled, applications MUST match each call to
  // 'SubtleMaybeSample()' returning true with a `SubtleConfirmSample()` call,
  // and use the result of the latter as the sampling decision.
  // In other words: the code should logically be equivalent to:
  //
  //    if (SubtleMaybeSample() && SubtleConfirmSample()) {
  //      // Sample this call
  //    }
  //
  // In the 'inline-size' optimized case, the `SubtleConfirmSample()` call can
  // be placed out of line, for example, the typical use case looks as follows:
  //
  //   // --- frobber.h -----------
  //   void FrobberSampled();
  //
  //   inline void FrobberImpl() {
  //     // ...
  //   }
  //
  //   inline void Frobber() {
  //     if (ABSL_PREDICT_FALSE(sampler.SubtleMaybeSample())) {
  //       FrobberSampled();
  //     } else {
  //       FrobberImpl();
  //     }
  //   }
  //
  //   // --- frobber.cc -----------
  //   void FrobberSampled() {
  //     if (!sampler.SubtleConfirmSample())) {
  //       // Spurious false positive
  //       FrobberImpl();
  //       return;
  //     }
  //
  //     // Sampled execution
  //     // ...
  //   }
  inline bool SubtleMaybeSample() noexcept;
  bool SubtleConfirmSample() noexcept;

 protected:
  // We explicitly don't use a virtual destructor as this class is never
  // virtually destroyed, and it keeps the class trivial, which avoids TLS
  // prologue and epilogue code for our TLS instances.
  ~PeriodicSamplerBase() = default;

  // Returns the next stride for our sampler.
  // This function is virtual for testing purposes only.
  virtual int64_t GetExponentialBiased(int period) noexcept;

 private:
  // Returns the current period of this sampler. Thread-safe.
  virtual int period() const noexcept = 0;

  int64_t stride_ = 0;
  ExponentialBiased rng_;
};

inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept {
  // We explicitly count up and not down, as the compiler does not generate
  // ideal code for counting down. See also https://gcc.godbolt.org/z/FTPDSM
  //
  // With `if (ABSL_PREDICT_FALSE(++stride_ < 0))`
  //    add     QWORD PTR fs:sampler@tpoff+8, 1
  //    jns     .L15
  //    ret
  //
  // With `if (ABSL_PREDICT_FALSE(--stride_ > 0))`
  //    mov     rax, QWORD PTR fs:sampler@tpoff+8
  //    sub     rax, 1
  //    mov     QWORD PTR fs:sampler@tpoff+8, rax
  //    test    rax, rax
  //    jle     .L15
  //    ret
  //    add     QWORD PTR fs:sampler@tpoff+8, 1
  //    jns     .L15
  //    ret
  //
  // --stride >= 0 does work, but makes our logic slightly harder as in that
  // case we have less convenient zero-init and over-run values.
  if (ABSL_PREDICT_FALSE(++stride_ < 0)) {
    return false;
  }
  return true;
}

inline bool PeriodicSamplerBase::Sample() noexcept {
  return ABSL_PREDICT_FALSE(SubtleMaybeSample()) ? SubtleConfirmSample()
                                                 : false;
}

// PeriodicSampler is a concreted periodic sampler implementation.
// The user provided Tag identifies the implementation, and is required to
// isolate the global state of this instance from other instances.
//
// Typical use case:
//
//   struct HashTablezTag {};
//   thread_local PeriodicSampler sampler;
//
//   void HashTableSamplingLogic(...) {
//     if (sampler.Sample()) {
//       HashTableSlowSamplePath(...);
//     }
//   }
//
template <typename Tag, int default_period = 0>
class PeriodicSampler final : public PeriodicSamplerBase {
 public:
  ~PeriodicSampler() = default;

  int period() const noexcept final {
    return period_.load(std::memory_order_relaxed);
  }

  // Sets the global period for this sampler. Thread-safe.
  // Setting a period of 0 disables the sampler, i.e., every call to Sample()
  // will return false. Setting a period of 1 puts the sampler in 'always on'
  // mode, i.e., every call to Sample() returns true.
  static void SetGlobalPeriod(int period) {
    period_.store(period, std::memory_order_relaxed);
  }

 private:
  static std::atomic<int> period_;
};

template <typename Tag, int default_period>
std::atomic<int> PeriodicSampler<Tag, default_period>::period_(default_period);

}  // namespace base_internal
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_