tdlight/tdutils/td/utils/MpmcWaiter.h

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//
2022-12-31 22:28:08 +01:00
// Copyright Aliaksei Levin (levlam@telegram.org), Arseny Smirnov (arseny30@gmail.com) 2014-2023
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#pragma once
#include "td/utils/common.h"
#include "td/utils/logging.h"
#include "td/utils/port/sleep.h"
#include <algorithm>
#include <atomic>
#include <condition_variable>
#include <mutex>
namespace td {
class MpmcEagerWaiter {
public:
struct Slot {
private:
friend class MpmcEagerWaiter;
int yields;
uint32 worker_id;
};
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static void init_slot(Slot &slot, uint32 worker_id) {
slot.yields = 0;
slot.worker_id = worker_id;
}
void wait(Slot &slot) {
if (slot.yields < RoundsTillSleepy) {
yield();
slot.yields++;
} else if (slot.yields == RoundsTillSleepy) {
auto state = state_.load(std::memory_order_relaxed);
if (!State::has_worker(state)) {
auto new_state = State::with_worker(state, slot.worker_id);
if (state_.compare_exchange_strong(state, new_state, std::memory_order_acq_rel)) {
yield();
slot.yields++;
return;
}
if (state == State::awake()) {
slot.yields = 0;
return;
}
}
yield();
slot.yields = 0;
} else if (slot.yields < RoundsTillAsleep) {
auto state = state_.load(std::memory_order_acquire);
if (State::still_sleepy(state, slot.worker_id)) {
yield();
slot.yields++;
return;
}
slot.yields = 0;
} else {
auto state = state_.load(std::memory_order_acquire);
if (State::still_sleepy(state, slot.worker_id)) {
std::unique_lock<std::mutex> lock(mutex_);
if (state_.compare_exchange_strong(state, State::asleep(), std::memory_order_acq_rel)) {
condition_variable_.wait(lock);
}
}
slot.yields = 0;
}
}
void stop_wait(Slot &slot) {
if (slot.yields > RoundsTillSleepy) {
notify_cold();
}
slot.yields = 0;
}
void close() {
}
void notify() {
std::atomic_thread_fence(std::memory_order_seq_cst);
if (state_.load(std::memory_order_acquire) == State::awake()) {
return;
}
notify_cold();
}
private:
struct State {
static constexpr uint32 awake() {
return 0;
}
static constexpr uint32 asleep() {
return 1;
}
static bool is_asleep(uint32 state) {
return (state & 1) != 0;
}
static bool has_worker(uint32 state) {
return (state >> 1) != 0;
}
static int32 with_worker(uint32 state, uint32 worker) {
return state | ((worker + 1) << 1);
}
static bool still_sleepy(uint32 state, uint32 worker) {
return (state >> 1) == (worker + 1);
}
};
enum { RoundsTillSleepy = 32, RoundsTillAsleep = 64 };
// enum { RoundsTillSleepy = 1, RoundsTillAsleep = 2 };
std::atomic<uint32> state_{State::awake()};
std::mutex mutex_;
std::condition_variable condition_variable_;
void notify_cold() {
auto old_state = state_.exchange(State::awake(), std::memory_order_release);
if (State::is_asleep(old_state)) {
std::lock_guard<std::mutex> guard(mutex_);
condition_variable_.notify_all();
}
}
static void yield() {
// whatever, this is better than sched_yield
usleep_for(1);
}
};
class MpmcSleepyWaiter {
public:
struct Slot {
private:
friend class MpmcSleepyWaiter;
enum State { Search, Work, Sleep } state_{Work};
void park() {
std::unique_lock<std::mutex> guard(mutex_);
condition_variable_.wait(guard, [&] { return unpark_flag_; });
unpark_flag_ = false;
}
bool cancel_park() {
auto res = unpark_flag_;
unpark_flag_ = false;
return res;
}
void unpark() {
//TODO: try to unlock guard before notify_all
std::unique_lock<std::mutex> guard(mutex_);
unpark_flag_ = true;
condition_variable_.notify_all();
}
std::mutex mutex_;
std::condition_variable condition_variable_;
bool unpark_flag_{false}; // TODO: move out of lock
int yield_cnt{0};
int32 worker_id{0};
public:
char padding[TD_CONCURRENCY_PAD];
};
// There are a lot of workers
// Each has a slot
//
// States of a worker:
// - searching for work | Search
// - processing work | Work
// - sleeping | Sleep
//
// When somebody adds a work it calls notify
//
// notify
// if there are workers in search phase do nothing.
// if all workers are awake do nothing
// otherwise wake some random worker
//
// Initially all workers are in Search mode.
//
// When worker found nothing it may try to call wait.
// This may put it in a Sleep for some time.
// After wait returns worker will be in Search state again.
//
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// If a worker found a work and ready to process it, then it may call stop_wait.
// This will cause transition from Search to Work state.
//
// Main invariant:
// After notify is called there should be at least on worker in Search or Work state.
// If possible - in Search state
//
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static void init_slot(Slot &slot, int32 worker_id) {
slot.state_ = Slot::State::Work;
slot.unpark_flag_ = false;
slot.worker_id = worker_id;
VLOG(waiter) << "Init slot " << worker_id;
}
static constexpr int VERBOSITY_NAME(waiter) = VERBOSITY_NAME(DEBUG) + 10;
void wait(Slot &slot) {
if (slot.state_ == Slot::State::Work) {
VLOG(waiter) << "Work -> Search";
state_++;
slot.state_ = Slot::State::Search;
slot.yield_cnt = 0;
return;
}
if (slot.state_ == Slot::Search) {
if (slot.yield_cnt++ < 10 && false) {
// TODO some sleep backoff is possible
return;
}
slot.state_ = Slot::State::Sleep;
std::unique_lock<std::mutex> guard(sleepers_mutex_);
auto state_view = StateView(state_.fetch_add((1 << PARKING_SHIFT) - 1));
CHECK(state_view.searching_count != 0);
bool should_search = state_view.searching_count == 1;
if (closed_) {
return;
}
sleepers_.push_back(&slot);
LOG_CHECK(slot.unpark_flag_ == false) << slot.worker_id;
VLOG(waiter) << "Add to sleepers " << slot.worker_id;
//guard.unlock();
if (should_search) {
VLOG(waiter) << "Search -> Search once, then Sleep ";
return;
}
VLOG(waiter) << "Search -> Sleep " << state_view.searching_count << " " << state_view.parked_count;
}
CHECK(slot.state_ == Slot::State::Sleep);
VLOG(waiter) << "Park " << slot.worker_id;
slot.park();
VLOG(waiter) << "Resume " << slot.worker_id;
slot.state_ = Slot::State::Search;
slot.yield_cnt = 0;
}
void stop_wait(Slot &slot) {
if (slot.state_ == Slot::State::Work) {
return;
}
if (slot.state_ == Slot::State::Sleep) {
VLOG(waiter) << "Search once, then Sleep -> Work/Search " << slot.worker_id;
slot.state_ = Slot::State::Work;
std::unique_lock<std::mutex> guard(sleepers_mutex_);
auto it = std::find(sleepers_.begin(), sleepers_.end(), &slot);
if (it != sleepers_.end()) {
sleepers_.erase(it);
VLOG(waiter) << "Remove from sleepers " << slot.worker_id;
state_.fetch_sub((1 << PARKING_SHIFT) - 1);
guard.unlock();
} else {
guard.unlock();
VLOG(waiter) << "Not in sleepers" << slot.worker_id;
CHECK(slot.cancel_park());
}
}
VLOG(waiter) << "Search once, then Sleep -> Work " << slot.worker_id;
slot.state_ = Slot::State::Search;
auto state_view = StateView(state_.fetch_sub(1));
CHECK(state_view.searching_count != 0);
CHECK(state_view.searching_count < 1000);
bool should_notify = state_view.searching_count == 1;
if (should_notify) {
VLOG(waiter) << "Notify others";
notify();
}
VLOG(waiter) << "Search -> Work ";
slot.state_ = Slot::State::Work;
}
void notify() {
auto view = StateView(state_.load());
//LOG(ERROR) << view.parked_count;
if (view.searching_count > 0 || view.parked_count == 0) {
VLOG(waiter) << "Ingore notify: " << view.searching_count << " " << view.parked_count;
return;
}
VLOG(waiter) << "Notify: " << view.searching_count << " " << view.parked_count;
std::unique_lock<std::mutex> guard(sleepers_mutex_);
view = StateView(state_.load());
if (view.searching_count > 0) {
VLOG(waiter) << "Skip notify: got searching";
return;
}
CHECK(view.parked_count == static_cast<int>(sleepers_.size()));
if (sleepers_.empty()) {
VLOG(waiter) << "Skip notify: no sleepers";
return;
}
auto sleeper = sleepers_.back();
sleepers_.pop_back();
state_.fetch_sub((1 << PARKING_SHIFT) - 1);
VLOG(waiter) << "Unpark " << sleeper->worker_id;
sleeper->unpark();
}
void close() {
StateView state(state_.load());
LOG_CHECK(state.parked_count == 0) << state.parked_count;
LOG_CHECK(state.searching_count == 0) << state.searching_count;
}
private:
static constexpr int32 PARKING_SHIFT = 16;
struct StateView {
int32 parked_count;
int32 searching_count;
explicit StateView(int32 x) {
parked_count = x >> PARKING_SHIFT;
searching_count = x & ((1 << PARKING_SHIFT) - 1);
}
};
std::atomic<int32> state_{0};
std::mutex sleepers_mutex_;
vector<Slot *> sleepers_;
bool closed_ = false;
};
using MpmcWaiter = MpmcSleepyWaiter;
} // namespace td