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-- 3a871e2cd854a46770a9416189953b2b38980bcf by Andy Getzendanner <durandal@google.com>: Import of CCTZ from GitHub. PiperOrigin-RevId: 280660534 -- f5a155fb056dcf68b089c21aebb5bf5159dc0eb3 by Tom Manshreck <shreck@google.com>: Nit: fix format of algorithm.h comments PiperOrigin-RevId: 280474139 -- 08b117a18353c0daf51b33b45100e5a007e5ab0c by Abseil Team <absl-team@google.com>: Allow absl::Notification::HasBeenNotified to be inlined. PiperOrigin-RevId: 280453271 -- 26d33dd2bad72945f1987f7b2669dbec3a6247e6 by Derek Mauro <dmauro@google.com>: Fix unused variable warnings on Windows PiperOrigin-RevId: 280324417 -- 907e9b23029be683af3b443bafd6a577a595922f by Abseil Team <absl-team@google.com>: PeriodicSampler implementation (for internal-use only) PiperOrigin-RevId: 280257168 GitOrigin-RevId: 3a871e2cd854a46770a9416189953b2b38980bcf Change-Id: I6e2978cae22a6adc41c0577e5c21355e9db7c8f5
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| // 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. | ||
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| #include "absl/base/internal/periodic_sampler.h" | ||
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| #include <atomic> | ||
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| #include "absl/base/internal/exponential_biased.h" | ||
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| namespace absl { | ||
| namespace base_internal { | ||
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| int64_t PeriodicSamplerBase::GetExponentialBiased(int period) noexcept { | ||
| return rng_.Get(period); | ||
| } | ||
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| bool PeriodicSamplerBase::SubtleConfirmSample() noexcept { | ||
| int current_period = period(); | ||
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| // Deal with period case 0 (always off) and 1 (always on) | ||
| if (ABSL_PREDICT_FALSE(current_period < 2)) { | ||
| stride_ = 0; | ||
| return current_period == 1; | ||
| } | ||
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| // Check if this is the first call to Sample() | ||
| if (ABSL_PREDICT_FALSE(stride_ == 1)) { | ||
| stride_ = -1 - GetExponentialBiased(current_period); | ||
| if (stride_ < -1) { | ||
| ++stride_; | ||
| return false; | ||
| } | ||
| } | ||
| stride_ = -1 - GetExponentialBiased(current_period); | ||
| return true; | ||
| } | ||
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| } // namespace base_internal | ||
| } // namespace absl |
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| // 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. | ||
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| #ifndef ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ | ||
| #define ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ | ||
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| #include <stdint.h> | ||
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| #include <atomic> | ||
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| #include "absl/base/internal/exponential_biased.h" | ||
| #include "absl/base/optimization.h" | ||
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| namespace absl { | ||
| namespace base_internal { | ||
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| // 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; | ||
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| // 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; | ||
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| // 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; | ||
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| 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; | ||
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| // Returns the next stride for our sampler. | ||
| // This function is virtual for testing purposes only. | ||
| virtual int64_t GetExponentialBiased(int period) noexcept; | ||
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| private: | ||
| // Returns the current period of this sampler. Thread-safe. | ||
| virtual int period() const noexcept = 0; | ||
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| int64_t stride_ = 0; | ||
| ExponentialBiased rng_; | ||
| }; | ||
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| 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; | ||
| } | ||
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| inline bool PeriodicSamplerBase::Sample() noexcept { | ||
| return ABSL_PREDICT_FALSE(SubtleMaybeSample()) ? SubtleConfirmSample() | ||
| : false; | ||
| } | ||
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| // 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; | ||
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| int period() const noexcept final { | ||
| return period_.load(std::memory_order_relaxed); | ||
| } | ||
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| // 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); | ||
| } | ||
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| private: | ||
| static std::atomic<int> period_; | ||
| }; | ||
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| template <typename Tag, int default_period> | ||
| std::atomic<int> PeriodicSampler<Tag, default_period>::period_(default_period); | ||
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| } // namespace base_internal | ||
| } // namespace absl | ||
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| #endif // ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ |
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