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Diffstat (limited to 'absl/base/internal/periodic_sampler.h')
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diff --git a/absl/base/internal/periodic_sampler.h b/absl/base/internal/periodic_sampler.h new file mode 100644 index 00000000..f8a86796 --- /dev/null +++ b/absl/base/internal/periodic_sampler.h @@ -0,0 +1,211 @@ +// 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 { +ABSL_NAMESPACE_BEGIN +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; + + // Keep and decrement stride_ as an unsigned integer, but compare the value + // to zero casted as a signed int. clang and msvc do not create optimum code + // if we use signed for the combined decrement and sign comparison. + // + // Below 3 alternative options, all compiles generate the best code + // using the unsigned increment <---> signed int comparison option. + // + // Option 1: + // int64_t stride_; + // if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... } + // + // GCC x64 (OK) : https://gcc.godbolt.org/z/R5MzzA + // GCC ppc (OK) : https://gcc.godbolt.org/z/z7NZAt + // Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd + // ICC x64 (OK) : https://gcc.godbolt.org/z/rE6s8W + // MSVC x64 (OK) : https://gcc.godbolt.org/z/ARMXqS + // + // Option 2: + // int64_t stride_ = 0; + // if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... } + // + // GCC x64 (OK) : https://gcc.godbolt.org/z/jSQxYK + // GCC ppc (OK) : https://gcc.godbolt.org/z/VJdYaA + // Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX + // ICC x64 (OK) : https://gcc.godbolt.org/z/4snaFd + // MSVC x64 (BAD): https://gcc.godbolt.org/z/BgnEKE + // + // Option 3: + // uint64_t stride_; + // if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... } + // + // GCC x64 (OK) : https://gcc.godbolt.org/z/bFbfPy + // GCC ppc (OK) : https://gcc.godbolt.org/z/S9KkUE + // Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4 + // ICC x64 (OK) : https://gcc.godbolt.org/z/ptTNfD + // MSVC x64 (OK) : https://gcc.godbolt.org/z/76j4-5 + uint64_t stride_ = 0; + ExponentialBiased rng_; +}; + +inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept { + // See comments on `stride_` for the unsigned increment / signed compare. + if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++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 +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ |