Web服务器的线程策略_reservedthreadexecutor-优快云博客

本文链接：https://blog.youkuaiyun.com/a860MHz/article/details/101296461

1. 简介

当前主流的Web服务器如Tomcat、Jetty均已采用nio作为默认的I/O模型。通常nio的线程模型中会有一组Acceptor线程用于接收客户端连接，一组Selector线程用于监听和处理I/O事件。当Selector检测到I/O事件后，是用同一个线程执行业务逻辑，还是将事件提交到一个业务线程组执行呢？不同的应用服务器，不同的场景下有着不同的策略。

本文将介绍几种主流的应用服务器的业务线程策略。

2. 原理分析

2.1 Tomcat

对于Tomcat来说情况较为简单：

LimitLatch作为连接信号量，负责控制最大连接数，达到阈值后，连接请求将被拒绝
Acceptor是一组线程，负责accept客户端请求，当客户端请求到达时，accept方法构建一个Channel对象，并将Channel对象交给Poller处理
Poller中维护了一个Selector线程，当检测到OP_READ或者OP_WRITE事件时，生成一个SocketProcessor对象，提交给Executor执行

从下文代码中可以看出，对于OP_READ和OP_WRITE事件，dispatch都是true，SocketProcessor对象都会被提交到线程池Executor执行。
AbstractEndpoint.java

    public boolean processSocket(SocketWrapperBase<S> socketWrapper,
            SocketEvent event, boolean dispatch) {
        try {
            if (socketWrapper == null) {
                return false;
            }
            SocketProcessorBase<S> sc = null;
            if (processorCache != null) {
                sc = processorCache.pop();
            }
            if (sc == null) {
                sc = createSocketProcessor(socketWrapper, event);
            } else {
                sc.reset(socketWrapper, event);
            }
            Executor executor = getExecutor();
            if (dispatch && executor != null) {
                executor.execute(sc);
            } else {
                sc.run();
            }
        } catch (RejectedExecutionException ree) {
            getLog().warn(sm.getString("endpoint.executor.fail", socketWrapper) , ree);
            return false;
        } catch (Throwable t) {
            ExceptionUtils.handleThrowable(t);
            // This means we got an OOM or similar creating a thread, or that
            // the pool and its queue are full
            getLog().error(sm.getString("endpoint.process.fail"), t);
            return false;
        }
        return true;
    }

AbstractEndpoint.java

                            if (sk.isReadable()) {
                                if (socketWrapper.readOperation != null) {
                                    if (!socketWrapper.readOperation.process()) {
                                        closeSocket = true;
                                    }
                                } else if (!processSocket(socketWrapper, SocketEvent.OPEN_READ, true)) {
                                    closeSocket = true;
                                }
                            }
                            if (!closeSocket && sk.isWritable()) {
                                if (socketWrapper.writeOperation != null) {
                                    if (!socketWrapper.writeOperation.process()) {
                                        closeSocket = true;
                                    }
                                } else if (!processSocket(socketWrapper, SocketEvent.OPEN_WRITE, true)) {
                                    closeSocket = true;
                                }
                            }

2.2 Jetty

Jetty对于业务线程的处理则要复杂一些，它可以采用多种策略：

ProduceConsume，任务生产者自己生产和执行任务，当任务耗时较长时，会严重影响Poller的执行效率
ProduceExecuteConsume，如果是非阻塞任务，则当前生产者执行；否则，开启提交Executor执行任务。该策略不能重复利用CPU缓存，且线程上下文切换代价较高
ExecuteProduceConsume，由生产者执行任务；但是如果任务类型是非阻塞任务，且生产者不处于pending状态，则提交Executor执行
EatWhatYouKill，EatWhatYouKill是Jetty对于ExecuteProduceConsume的优化，依赖于QTP在9.4.x新增的ReservedThreadExecutor能力。当任务为非阻塞时，使用ProduceConsume模式；当任务为阻塞时，且生产者处于pending状态，使用EXECUTE_PRODUCE_CONSUME模式；生产者非pending状态，且ReservedThreadExecutor有可用线程时，也是用EXECUTE_PRODUCE_CONSUME模式；其他情况下使用PRODUCE_EXECUTE_CONSUME模式。

2.2.1 ReservedThreadExecutor

ReservedThreadExecutor是Jetty 9.4.x中对于QTP的增强，通过该Executor，Jetty可以在QTP中保留多个线程。

Jetty启动时

QueuedThreadPool.java

    @Override
    protected void doStart() throws Exception
    {
        // 默认情况下，_reservedThreads为-1
        if (_reservedThreads == 0)
        {
            _tryExecutor = NO_TRY;
        }
        else
        {
            ReservedThreadExecutor reserved = new ReservedThreadExecutor(this, _reservedThreads);
            reserved.setIdleTimeout(_idleTimeout, TimeUnit.MILLISECONDS);
            _tryExecutor = reserved;
        }
        addBean(_tryExecutor);

        super.doStart();
        // The threads count set to MIN_VALUE is used to signal to Runners that the pool is stopped.
        _counts.set(0, 0); // threads, idle
        ensureThreads();
    }

ReservedThreadExecutor.java

    public ReservedThreadExecutor(Executor executor, int capacity)
    {
        _executor = executor;
        _capacity = reservedThreads(executor, capacity);
       // 此处使用了无锁队列，以CPU时间为代价，提高并发性能
        _stack = new ConcurrentLinkedDeque<>();

        LOG.debug("{}", this);
    }

    // QTP实现了ThreadPool.SizedThreadPool接口，因此保留线程数在cpu核数和线程池size/10中取小
    private static int reservedThreads(Executor executor, int capacity)
    {
        if (capacity >= 0)
            return capacity;
        int cpus = ProcessorUtils.availableProcessors();
        if (executor instanceof ThreadPool.SizedThreadPool)
        {
            int threads = ((ThreadPool.SizedThreadPool)executor).getMaxThreads();
            return Math.max(1, Math.min(cpus, threads / 10));
        }
        return cpus;
    }

   // tryExecute方法是，TryExecutor接口定义的方法，调用方可以尝试执行一个runnable
   // 如果执行失败，可能是没有空闲保留线程，则返回false，任务不会提交给线程执行，也不会加入等待队列
    @Override
    public boolean tryExecute(Runnable task)
    {
        if (LOG.isDebugEnabled())
            LOG.debug("{} tryExecute {}", this, task);

        if (task == null)
            return false;

        ReservedThread thread = _stack.pollFirst();
        if (thread == null)
        {
            if (task != STOP)
                startReservedThread();
            return false;
        }

        int size = _size.decrementAndGet();
        thread.offer(task);

        if (size == 0 && task != STOP)
            startReservedThread();

        return true;
    }