lzx404243 commented on code in PR #9394: URL: https://github.com/apache/trafficserver/pull/9394#discussion_r1143603357
########## include/tscpp/util/Bravo.h: ########## @@ -0,0 +1,375 @@ +/** @file + + Implementation of BRAVO - Biased Locking for Reader-Writer Locks + + Dave Dice and Alex Kogan. 2019. BRAVO: Biased Locking for Reader-Writer Locks. + In Proceedings of the 2019 USENIX Annual Technical Conference (ATC). USENIX Association, Renton, WA, 315–328. + + https://www.usenix.org/conference/atc19/presentation/dice + + > Section 3. + > BRAVO acts as an accelerator layer, as readers can always fall back to the traditional underlying lock to gain read access. + > ... + > Write performance and the scalability of read-vs-write and write-vs-write behavior depends solely on the underlying lock. + + This code is C++ version of puzpuzpuz/xsync's RBMutex + https://github.com/puzpuzpuz/xsync/blob/main/rbmutex.go + Copyright (c) 2021 Andrey Pechkurov + + @section license License + + Licensed to the Apache Software Foundation (ASF) under one + or more contributor license agreements. See the NOTICE file + distributed with this work for additional information + regarding copyright ownership. The ASF licenses this file + to you 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. + */ + +#pragma once + +#include "DenseThreadId.h" + +#include "tscore/Diags.h" + +#include <array> +#include <atomic> +#include <cassert> +#include <chrono> +#include <shared_mutex> +#include <thread> + +namespace ts::bravo +{ +using time_point = std::chrono::time_point<std::chrono::system_clock>; + +#ifdef __cpp_lib_hardware_interference_size +using std::hardware_constructive_interference_size; +#else +// 64 bytes on x86-64 │ L1_CACHE_BYTES │ L1_CACHE_SHIFT │ __cacheline_aligned │ ... +constexpr std::size_t hardware_constructive_interference_size = 64; +#endif + +/** + ts::bravo::Token + + Token for readers. + 0 is special value that represents inital/invalid value. + */ +using Token = size_t; + +/** + ts::bravo::shared_lock + */ +template <class Mutex> class shared_lock +{ +public: + using mutex_type = Mutex; + + shared_lock() = default; + shared_lock(Mutex &m) : _mutex(&m) { lock(); } + + ~shared_lock() + { + if (_owns) { + _mutex->unlock_shared(_token); + } + }; + + //// + // Not Copyable + // + shared_lock(shared_lock const &) = delete; + shared_lock &operator=(shared_lock const &) = delete; + + //// + // Moveable + // + shared_lock(shared_lock &&s) : _mutex(s._mutex), _token(s._token), _owns(s._owns) + { + s._mutex = nullptr; + s._token = 0; + s._owns = false; + }; + + shared_lock & + operator=(shared_lock &&s) + { + if (_owns) { + _mutex->unlock_shared(_token); + } + _mutex = s._mutex; + _token = s._token; + _owns = s._owns; + + s._mutex = nullptr; + s._token = 0; + s._owns = false; + }; + + //// + // Shared locking + // + void + lock() + { + _mutex->lock_shared(_token); + _owns = true; + } + + bool + try_lock() + { + _owns = _mutex->try_lock_shared(_token); + return _owns; + } + + // not implemented yet + bool try_lock_for() = delete; + bool try_lock_until() = delete; + + void + unlock() + { + _mutex->unlock_shared(_token); + _owns = false; + } + + //// + // Modifiers + // + void + swap(shared_lock &s) + { + std::swap(_mutex, s._mutex); + std::swap(_token, s._token); + std::swap(_owns, s._owns); + } + + mutex_type * + release() + { + mutex_type *m = _mutex; + _mutex = nullptr; + _token = 0; + _owns = false; + return m; + } + + //// + // Observers + // + mutex_type * + mutex() + { + return _mutex; + } + + Token + token() + { + return _token; + } + + bool + owns_lock() + { + return _owns; + } + +private: + mutex_type *_mutex = nullptr; + Token _token = 0; + bool _owns = false; +}; + +/** + ts::bravo::shared_mutex + + You can use std::lock_guard for writers but, you can't use std::shared_lock for readers to handle ts::bravo::Token. + Use ts::bravo::shared_lock for readers. + + Set the SLOT_SIZE larger than DenseThreadId::num_possible_values to go fast-path. + */ +template <typename T = std::shared_mutex, size_t SLOT_SIZE = 256, int SLOWDOWN_GUARD = 7> class shared_mutex_impl +{ +public: + shared_mutex_impl() = default; + ~shared_mutex_impl() = default; + + //// + // No copying or moving. + // + shared_mutex_impl(shared_mutex_impl const &) = delete; + shared_mutex_impl &operator=(shared_mutex_impl const &) = delete; + + shared_mutex_impl(shared_mutex_impl &&) = delete; + shared_mutex_impl &operator=(shared_mutex_impl &&) = delete; + + //// + // Exclusive locking + // + void + lock() + { + _mutex.underlying.lock(); + _revoke(); + } + + bool + try_lock() + { + bool r = _mutex.underlying.try_lock(); + if (!r) { + return false; + } + + _revoke(); + + return true; + } + + void + unlock() + { + _mutex.underlying.unlock(); + } + + //// + // Shared locking + // + void + lock_shared(Token &token) + { + // Fast path + if (_mutex.read_bias.load(std::memory_order_acquire)) { + size_t index = DenseThreadId::self(); + for (size_t i = 0; i < SLOT_SIZE; ++i) { Review Comment: Not sure why there is a for loop in the locking path, although it can be rare for it to iterate more than once as it stops once there is no collision. Not sure how well this performs compared to just falling back to the slow path upon a collision, as in the BRAVO paper. The latter seems to have more predictable performance characteristics, as each thread can only go to one slot in the table(versus every thread can go to any slots which can increase future chance of collision). btw, in your current implementation with the use of `DenseThreadId`, will there be any collision at all? -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
