maurice/snix
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Export of internal Abseil changes

Browse source 
--
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
This commit is contained in:
Abseil Team 2019-11-15 08:07:12 -08:00 • committed by Andy Getz
parent fa8c75182f
commit 3df7b52a6a
12 changed files with 581 additions and 19 deletions
Download patch file Download diff file Expand all files Collapse all files

193
absl/base/internal/periodic_sampler.h Normal file
View file

@ -0,0 +1,193 @@
// 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_