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authorGravatar Abseil Team <absl-team@google.com>2020-02-25 22:27:31 +0100
committerGravatar CJ Johnson <johnsoncj@google.com>2020-02-25 17:56:58 -0500
commitb832dce8489ef7b6231384909fd9b68d5a5ff2b7 (patch)
tree3ad4be9a9a4105366be714da9458e076a77be18f /absl/base/internal/periodic_sampler.h
parentaa844899c937bde5d2b24f276b59997e5b668bde (diff)
Creation of LTS branch "lts_2020_02_25"20200225
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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.
+
+#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_
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