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// Copyright 2018 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.
//
// -----------------------------------------------------------------------------
// File: hashtablez_sampler.h
// -----------------------------------------------------------------------------
//
// This header file defines the API for a low level library to sample hashtables
// and collect runtime statistics about them.
//
// `HashtablezSampler` controls the lifecycle of `HashtablezInfo` objects which
// store information about a single sample.
//
// `Record*` methods store information into samples.
// `Sample()` and `Unsample()` make use of a single global sampler with
// properties controlled by the flags hashtablez_enabled,
// hashtablez_sample_rate, and hashtablez_max_samples.
//
// WARNING
//
// Using this sampling API may cause sampled Swiss tables to use the global
// allocator (operator `new`) in addition to any custom allocator. If you
// are using a table in an unusual circumstance where allocation or calling a
// linux syscall is unacceptable, this could interfere.
//
// This utility is internal-only. Use at your own risk.
#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
#define ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
#include <atomic>
#include <functional>
#include <memory>
#include <vector>
#include "absl/base/internal/per_thread_tls.h"
#include "absl/base/optimization.h"
#include "absl/container/internal/have_sse.h"
#include "absl/profiling/internal/sample_recorder.h"
#include "absl/synchronization/mutex.h"
#include "absl/utility/utility.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace container_internal {
// Stores information about a sampled hashtable. All mutations to this *must*
// be made through `Record*` functions below. All reads from this *must* only
// occur in the callback to `HashtablezSampler::Iterate`.
struct HashtablezInfo : public profiling_internal::Sample<HashtablezInfo> {
// Constructs the object but does not fill in any fields.
HashtablezInfo();
~HashtablezInfo();
HashtablezInfo(const HashtablezInfo&) = delete;
HashtablezInfo& operator=(const HashtablezInfo&) = delete;
// Puts the object into a clean state, fills in the logically `const` members,
// blocking for any readers that are currently sampling the object.
void PrepareForSampling() ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);
// These fields are mutated by the various Record* APIs and need to be
// thread-safe.
std::atomic<size_t> capacity;
std::atomic<size_t> size;
std::atomic<size_t> num_erases;
std::atomic<size_t> num_rehashes;
std::atomic<size_t> max_probe_length;
std::atomic<size_t> total_probe_length;
std::atomic<size_t> hashes_bitwise_or;
std::atomic<size_t> hashes_bitwise_and;
std::atomic<size_t> hashes_bitwise_xor;
std::atomic<size_t> max_reserve;
// All of the fields below are set by `PrepareForSampling`, they must not be
// mutated in `Record*` functions. They are logically `const` in that sense.
// These are guarded by init_mu, but that is not externalized to clients, who
// can only read them during `HashtablezSampler::Iterate` which will hold the
// lock.
static constexpr int kMaxStackDepth = 64;
absl::Time create_time;
int32_t depth;
void* stack[kMaxStackDepth];
};
inline void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
#if ABSL_INTERNAL_RAW_HASH_SET_HAVE_SSE2
total_probe_length /= 16;
#else
total_probe_length /= 8;
#endif
info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
info->num_erases.store(0, std::memory_order_relaxed);
// There is only one concurrent writer, so `load` then `store` is sufficient
// instead of using `fetch_add`.
info->num_rehashes.store(
1 + info->num_rehashes.load(std::memory_order_relaxed),
std::memory_order_relaxed);
}
inline void RecordReservationSlow(HashtablezInfo* info,
size_t target_capacity) {
info->max_reserve.store(
(std::max)(info->max_reserve.load(std::memory_order_relaxed),
target_capacity),
std::memory_order_relaxed);
}
inline void RecordClearedReservationSlow(HashtablezInfo* info) {
info->max_reserve.store(0, std::memory_order_relaxed);
}
inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
size_t capacity) {
info->size.store(size, std::memory_order_relaxed);
info->capacity.store(capacity, std::memory_order_relaxed);
if (size == 0) {
// This is a clear, reset the total/num_erases too.
info->total_probe_length.store(0, std::memory_order_relaxed);
info->num_erases.store(0, std::memory_order_relaxed);
}
}
void RecordInsertSlow(HashtablezInfo* info, size_t hash,
size_t distance_from_desired);
inline void RecordEraseSlow(HashtablezInfo* info) {
info->size.fetch_sub(1, std::memory_order_relaxed);
// There is only one concurrent writer, so `load` then `store` is sufficient
// instead of using `fetch_add`.
info->num_erases.store(
1 + info->num_erases.load(std::memory_order_relaxed),
std::memory_order_relaxed);
}
HashtablezInfo* SampleSlow(int64_t* next_sample);
void UnsampleSlow(HashtablezInfo* info);
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
#error ABSL_INTERNAL_HASHTABLEZ_SAMPLE cannot be directly set
#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
class HashtablezInfoHandle {
public:
explicit HashtablezInfoHandle() : info_(nullptr) {}
explicit HashtablezInfoHandle(HashtablezInfo* info) : info_(info) {}
~HashtablezInfoHandle() {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
UnsampleSlow(info_);
}
HashtablezInfoHandle(const HashtablezInfoHandle&) = delete;
HashtablezInfoHandle& operator=(const HashtablezInfoHandle&) = delete;
HashtablezInfoHandle(HashtablezInfoHandle&& o) noexcept
: info_(absl::exchange(o.info_, nullptr)) {}
HashtablezInfoHandle& operator=(HashtablezInfoHandle&& o) noexcept {
if (ABSL_PREDICT_FALSE(info_ != nullptr)) {
UnsampleSlow(info_);
}
info_ = absl::exchange(o.info_, nullptr);
return *this;
}
inline void RecordStorageChanged(size_t size, size_t capacity) {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
RecordStorageChangedSlow(info_, size, capacity);
}
inline void RecordRehash(size_t total_probe_length) {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
RecordRehashSlow(info_, total_probe_length);
}
inline void RecordReservation(size_t target_capacity) {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
RecordReservationSlow(info_, target_capacity);
}
inline void RecordClearedReservation() {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
RecordClearedReservationSlow(info_);
}
inline void RecordInsert(size_t hash, size_t distance_from_desired) {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
RecordInsertSlow(info_, hash, distance_from_desired);
}
inline void RecordErase() {
if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
RecordEraseSlow(info_);
}
friend inline void swap(HashtablezInfoHandle& lhs,
HashtablezInfoHandle& rhs) {
std::swap(lhs.info_, rhs.info_);
}
private:
friend class HashtablezInfoHandlePeer;
HashtablezInfo* info_;
};
#else
// Ensure that when Hashtablez is turned off at compile time, HashtablezInfo can
// be removed by the linker, in order to reduce the binary size.
class HashtablezInfoHandle {
public:
explicit HashtablezInfoHandle() = default;
explicit HashtablezInfoHandle(std::nullptr_t) {}
inline void RecordStorageChanged(size_t /*size*/, size_t /*capacity*/) {}
inline void RecordRehash(size_t /*total_probe_length*/) {}
inline void RecordReservation(size_t /*target_capacity*/) {}
inline void RecordClearedReservation() {}
inline void RecordInsert(size_t /*hash*/, size_t /*distance_from_desired*/) {}
inline void RecordErase() {}
friend inline void swap(HashtablezInfoHandle& /*lhs*/,
HashtablezInfoHandle& /*rhs*/) {}
};
#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
// Returns an RAII sampling handle that manages registration and unregistation
// with the global sampler.
inline HashtablezInfoHandle Sample() {
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
if (ABSL_PREDICT_TRUE(--global_next_sample > 0)) {
return HashtablezInfoHandle(nullptr);
}
return HashtablezInfoHandle(SampleSlow(&global_next_sample));
#else
return HashtablezInfoHandle(nullptr);
#endif // !ABSL_PER_THREAD_TLS
}
using HashtablezSampler =
::absl::profiling_internal::SampleRecorder<HashtablezInfo>;
// Returns a global Sampler.
HashtablezSampler& GlobalHashtablezSampler();
// Enables or disables sampling for Swiss tables.
void SetHashtablezEnabled(bool enabled);
// Sets the rate at which Swiss tables will be sampled.
void SetHashtablezSampleParameter(int32_t rate);
// Sets a soft max for the number of samples that will be kept.
void SetHashtablezMaxSamples(int32_t max);
// Configuration override.
// This allows process-wide sampling without depending on order of
// initialization of static storage duration objects.
// The definition of this constant is weak, which allows us to inject a
// different value for it at link time.
extern "C" bool ABSL_INTERNAL_C_SYMBOL(AbslContainerInternalSampleEverything)();
} // namespace container_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_