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authorGravatar Abseil Team <absl-team@google.com>2019-11-15 08:07:12 -0800
committerGravatar Andy Getz <durandal@google.com>2019-11-15 11:16:16 -0500
commit3df7b52a6ada51a72a23796b844549a7b282f1b8 (patch)
tree12f9fb49f9c955408628aafaf9448ecaf357ee8d /absl/base/internal/periodic_sampler.h
parentfa8c75182fbfdeddb2485fc0d53baeda3f40b7a3 (diff)
Export of internal Abseil changes
-- 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.
+
+#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_