// 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
//
// http://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.
//
// This is 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.
#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/optimization.h"
#include "absl/synchronization/mutex.h"
#include "absl/utility/utility.h"
namespace absl {
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 {
// 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() 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> max_probe_length;
std::atomic<size_t> total_probe_length;
std::atomic<size_t> hashes_bitwise_or;
std::atomic<size_t> hashes_bitwise_and;
// `HashtablezSampler` maintains intrusive linked lists for all samples. See
// comments on `HashtablezSampler::all_` for details on these. `init_mu`
// guards the ability to restore the sample to a pristine state. This
// prevents races with sampling and resurrecting an object.
absl::Mutex init_mu;
HashtablezInfo* next;
HashtablezInfo* dead GUARDED_BY(init_mu);
// 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 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);
}
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);
info->num_erases.fetch_add(1, std::memory_order_relaxed);
}
HashtablezInfo* SampleSlow(int64_t* next_sample);
void UnsampleSlow(HashtablezInfo* info);
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 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_;
};
// Returns an RAII sampling handle that manages registration and unregistation
// with the global sampler.
inline HashtablezInfoHandle Sample() {
#if ABSL_HAVE_THREAD_LOCAL
thread_local int64_t next_sample = 0;
#else // ABSL_HAVE_THREAD_LOCAL
static auto* mu = new absl::Mutex;
static int64_t next_sample = 0;
absl::MutexLock l(mu);
#endif // ABSL_HAVE_THREAD_LOCAL
if (ABSL_PREDICT_TRUE(--next_sample > 0)) {
return HashtablezInfoHandle(nullptr);
}
return HashtablezInfoHandle(SampleSlow(&next_sample));
}
// Holds samples and their associated stack traces with a soft limit of
// `SetHashtablezMaxSamples()`.
//
// Thread safe.
class HashtablezSampler {
public:
// Returns a global Sampler.
static HashtablezSampler& Global();
HashtablezSampler();
~HashtablezSampler();
// Registers for sampling. Returns an opaque registration info.
HashtablezInfo* Register();
// Unregisters the sample.
void Unregister(HashtablezInfo* sample);
// Iterates over all the registered `StackInfo`s. Returning the number of
// samples that have been dropped.
int64_t Iterate(const std::function<void(const HashtablezInfo& stack)>& f);
private:
void PushNew(HashtablezInfo* sample);
void PushDead(HashtablezInfo* sample);
HashtablezInfo* PopDead();
std::atomic<size_t> dropped_samples_;
std::atomic<size_t> size_estimate_;
// Intrusive lock free linked lists for tracking samples.
//
// `all_` records all samples (they are never removed from this list) and is
// terminated with a `nullptr`.
//
// `graveyard_.dead` is a circular linked list. When it is empty,
// `graveyard_.dead == &graveyard`. The list is circular so that
// every item on it (even the last) has a non-null dead pointer. This allows
// `Iterate` to determine if a given sample is live or dead using only
// information on the sample itself.
//
// For example, nodes [A, B, C, D, E] with [A, C, E] alive and [B, D] dead
// looks like this (G is the Graveyard):
//
// +---+ +---+ +---+ +---+ +---+
// all -->| A |--->| B |--->| C |--->| D |--->| E |
// | | | | | | | | | |
// +---+ | | +->| |-+ | | +->| |-+ | |
// | G | +---+ | +---+ | +---+ | +---+ | +---+
// | | | | | |
// | | --------+ +--------+ |
// +---+ |
// ^ |
// +--------------------------------------+
//
std::atomic<HashtablezInfo*> all_;
HashtablezInfo graveyard_;
};
// 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);
} // namespace container_internal
} // namespace absl
#endif // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
