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// Copyright 2018 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// -----------------------------------------------------------------------------
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// File: hashtablez_sampler.h
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// -----------------------------------------------------------------------------
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//
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// This header file defines the API for a low level library to sample hashtables
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// and collect runtime statistics about them.
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//
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// `HashtablezSampler` controls the lifecycle of `HashtablezInfo` objects which
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// store information about a single sample.
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//
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// `Record*` methods store information into samples.
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// `Sample()` and `Unsample()` make use of a single global sampler with
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// properties controlled by the flags hashtablez_enabled,
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// hashtablez_sample_rate, and hashtablez_max_samples.
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//
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// WARNING
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//
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// Using this sampling API may cause sampled Swiss tables to use the global
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// allocator (operator `new`) in addition to any custom allocator. If you
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// are using a table in an unusual circumstance where allocation or calling a
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// linux syscall is unacceptable, this could interfere.
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//
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// This utility is internal-only. Use at your own risk.
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#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
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#define ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
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#include <atomic>
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#include <functional>
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#include <memory>
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#include <vector>
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#include "absl/base/internal/per_thread_tls.h"
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#include "absl/base/optimization.h"
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#include "absl/container/internal/have_sse.h"
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#include "absl/synchronization/mutex.h"
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#include "absl/utility/utility.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace container_internal {
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// Stores information about a sampled hashtable. All mutations to this *must*
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// be made through `Record*` functions below. All reads from this *must* only
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// occur in the callback to `HashtablezSampler::Iterate`.
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struct HashtablezInfo {
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// Constructs the object but does not fill in any fields.
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HashtablezInfo();
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~HashtablezInfo();
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HashtablezInfo(const HashtablezInfo&) = delete;
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HashtablezInfo& operator=(const HashtablezInfo&) = delete;
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// Puts the object into a clean state, fills in the logically `const` members,
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// blocking for any readers that are currently sampling the object.
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void PrepareForSampling() ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);
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// These fields are mutated by the various Record* APIs and need to be
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// thread-safe.
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std::atomic<size_t> capacity;
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std::atomic<size_t> size;
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std::atomic<size_t> num_erases;
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std::atomic<size_t> num_rehashes;
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std::atomic<size_t> max_probe_length;
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std::atomic<size_t> total_probe_length;
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std::atomic<size_t> hashes_bitwise_or;
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std::atomic<size_t> hashes_bitwise_and;
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std::atomic<size_t> hashes_bitwise_xor;
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// `HashtablezSampler` maintains intrusive linked lists for all samples. See
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// comments on `HashtablezSampler::all_` for details on these. `init_mu`
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// guards the ability to restore the sample to a pristine state. This
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// prevents races with sampling and resurrecting an object.
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absl::Mutex init_mu;
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HashtablezInfo* next;
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HashtablezInfo* dead ABSL_GUARDED_BY(init_mu);
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// All of the fields below are set by `PrepareForSampling`, they must not be
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// mutated in `Record*` functions. They are logically `const` in that sense.
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// These are guarded by init_mu, but that is not externalized to clients, who
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// can only read them during `HashtablezSampler::Iterate` which will hold the
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// lock.
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static constexpr int kMaxStackDepth = 64;
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absl::Time create_time;
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int32_t depth;
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void* stack[kMaxStackDepth];
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};
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inline void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
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#if ABSL_INTERNAL_RAW_HASH_SET_HAVE_SSE2
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total_probe_length /= 16;
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#else
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total_probe_length /= 8;
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#endif
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info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
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info->num_erases.store(0, std::memory_order_relaxed);
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// There is only one concurrent writer, so `load` then `store` is sufficient
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// instead of using `fetch_add`.
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info->num_rehashes.store(
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1 + info->num_rehashes.load(std::memory_order_relaxed),
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std::memory_order_relaxed);
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}
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inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
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size_t capacity) {
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info->size.store(size, std::memory_order_relaxed);
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info->capacity.store(capacity, std::memory_order_relaxed);
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if (size == 0) {
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// This is a clear, reset the total/num_erases too.
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info->total_probe_length.store(0, std::memory_order_relaxed);
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info->num_erases.store(0, std::memory_order_relaxed);
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}
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}
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void RecordInsertSlow(HashtablezInfo* info, size_t hash,
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size_t distance_from_desired);
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inline void RecordEraseSlow(HashtablezInfo* info) {
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info->size.fetch_sub(1, std::memory_order_relaxed);
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// There is only one concurrent writer, so `load` then `store` is sufficient
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// instead of using `fetch_add`.
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info->num_erases.store(
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1 + info->num_erases.load(std::memory_order_relaxed),
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std::memory_order_relaxed);
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}
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HashtablezInfo* SampleSlow(int64_t* next_sample);
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void UnsampleSlow(HashtablezInfo* info);
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#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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#error ABSL_INTERNAL_HASHTABLEZ_SAMPLE cannot be directly set
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#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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class HashtablezInfoHandle {
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public:
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explicit HashtablezInfoHandle() : info_(nullptr) {}
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explicit HashtablezInfoHandle(HashtablezInfo* info) : info_(info) {}
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~HashtablezInfoHandle() {
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if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
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UnsampleSlow(info_);
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}
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HashtablezInfoHandle(const HashtablezInfoHandle&) = delete;
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HashtablezInfoHandle& operator=(const HashtablezInfoHandle&) = delete;
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HashtablezInfoHandle(HashtablezInfoHandle&& o) noexcept
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: info_(absl::exchange(o.info_, nullptr)) {}
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HashtablezInfoHandle& operator=(HashtablezInfoHandle&& o) noexcept {
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if (ABSL_PREDICT_FALSE(info_ != nullptr)) {
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UnsampleSlow(info_);
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}
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info_ = absl::exchange(o.info_, nullptr);
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return *this;
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}
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inline void RecordStorageChanged(size_t size, size_t capacity) {
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if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
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RecordStorageChangedSlow(info_, size, capacity);
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}
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inline void RecordRehash(size_t total_probe_length) {
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if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
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RecordRehashSlow(info_, total_probe_length);
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}
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inline void RecordInsert(size_t hash, size_t distance_from_desired) {
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if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
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RecordInsertSlow(info_, hash, distance_from_desired);
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}
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inline void RecordErase() {
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if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
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RecordEraseSlow(info_);
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}
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friend inline void swap(HashtablezInfoHandle& lhs,
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HashtablezInfoHandle& rhs) {
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std::swap(lhs.info_, rhs.info_);
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}
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private:
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friend class HashtablezInfoHandlePeer;
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HashtablezInfo* info_;
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};
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#else
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// Ensure that when Hashtablez is turned off at compile time, HashtablezInfo can
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// be removed by the linker, in order to reduce the binary size.
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class HashtablezInfoHandle {
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public:
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explicit HashtablezInfoHandle() = default;
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explicit HashtablezInfoHandle(std::nullptr_t) {}
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inline void RecordStorageChanged(size_t /*size*/, size_t /*capacity*/) {}
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inline void RecordRehash(size_t /*total_probe_length*/) {}
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inline void RecordInsert(size_t /*hash*/, size_t /*distance_from_desired*/) {}
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inline void RecordErase() {}
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friend inline void swap(HashtablezInfoHandle& /*lhs*/,
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HashtablezInfoHandle& /*rhs*/) {}
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};
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#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
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#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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// Returns an RAII sampling handle that manages registration and unregistation
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// with the global sampler.
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inline HashtablezInfoHandle Sample() {
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#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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if (ABSL_PREDICT_TRUE(--global_next_sample > 0)) {
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return HashtablezInfoHandle(nullptr);
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}
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return HashtablezInfoHandle(SampleSlow(&global_next_sample));
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#else
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return HashtablezInfoHandle(nullptr);
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#endif // !ABSL_PER_THREAD_TLS
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}
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// Holds samples and their associated stack traces with a soft limit of
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// `SetHashtablezMaxSamples()`.
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//
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// Thread safe.
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class HashtablezSampler {
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public:
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// Returns a global Sampler.
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static HashtablezSampler& Global();
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HashtablezSampler();
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~HashtablezSampler();
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// Registers for sampling. Returns an opaque registration info.
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HashtablezInfo* Register();
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// Unregisters the sample.
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void Unregister(HashtablezInfo* sample);
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// The dispose callback will be called on all samples the moment they are
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// being unregistered. Only affects samples that are unregistered after the
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// callback has been set.
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// Returns the previous callback.
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using DisposeCallback = void (*)(const HashtablezInfo&);
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DisposeCallback SetDisposeCallback(DisposeCallback f);
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// Iterates over all the registered `StackInfo`s. Returning the number of
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// samples that have been dropped.
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int64_t Iterate(const std::function<void(const HashtablezInfo& stack)>& f);
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private:
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void PushNew(HashtablezInfo* sample);
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void PushDead(HashtablezInfo* sample);
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HashtablezInfo* PopDead();
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std::atomic<size_t> dropped_samples_;
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std::atomic<size_t> size_estimate_;
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// Intrusive lock free linked lists for tracking samples.
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//
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// `all_` records all samples (they are never removed from this list) and is
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// terminated with a `nullptr`.
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//
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// `graveyard_.dead` is a circular linked list. When it is empty,
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// `graveyard_.dead == &graveyard`. The list is circular so that
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// every item on it (even the last) has a non-null dead pointer. This allows
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// `Iterate` to determine if a given sample is live or dead using only
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// information on the sample itself.
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//
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// For example, nodes [A, B, C, D, E] with [A, C, E] alive and [B, D] dead
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// looks like this (G is the Graveyard):
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//
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// +---+ +---+ +---+ +---+ +---+
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// all -->| A |--->| B |--->| C |--->| D |--->| E |
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// | | | | | | | | | |
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// +---+ | | +->| |-+ | | +->| |-+ | |
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// | G | +---+ | +---+ | +---+ | +---+ | +---+
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// | | | | | |
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// | | --------+ +--------+ |
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// +---+ |
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// ^ |
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// +--------------------------------------+
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//
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std::atomic<HashtablezInfo*> all_;
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HashtablezInfo graveyard_;
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std::atomic<DisposeCallback> dispose_;
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};
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// Enables or disables sampling for Swiss tables.
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void SetHashtablezEnabled(bool enabled);
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// Sets the rate at which Swiss tables will be sampled.
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void SetHashtablezSampleParameter(int32_t rate);
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// Sets a soft max for the number of samples that will be kept.
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void SetHashtablezMaxSamples(int32_t max);
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// Configuration override.
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// This allows process-wide sampling without depending on order of
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// initialization of static storage duration objects.
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// The definition of this constant is weak, which allows us to inject a
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// different value for it at link time.
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extern "C" bool ABSL_INTERNAL_C_SYMBOL(AbslContainerInternalSampleEverything)();
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} // namespace container_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
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