java - 非阻塞 I/O 与使用线程(上下文切换有多糟糕？)

Question

我们在我工作的一个程序中经常使用套接字，我们有时同时处理来自多达大约 100 台机器的连接。我们有一个非阻塞的组合 I/O与状态表一起使用来管理它和使用线程的传统 Java 套接字。

我们在非阻塞套接字方面遇到了很多问题，我个人更喜欢使用线程来更好地处理套接字。所以我的问题是:

在单个线程上使用非阻塞套接字可以节省多少？使用线程涉及的上下文切换有多糟糕？您可以扩展多少个并发连接以使用 Java 中的线程模型？

Answer 1

I/O 和非阻塞 I/O 选择取决于您的服务器 Activity 配置文件。例如。如果您使用长期连接和数千个客户端，则 I/O 可能会因为系统资源耗尽而变得过于昂贵。但是，不会挤占 CPU 缓存的直接 I/O 比非阻塞 I/O 更快。有一篇关于此的好文章 - Writing Java Multithreaded Servers - whats old is new .

关于上下文切换成本——这是芯片操作。考虑下面的简单测试:

package com;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

public class AAA {

    private static final long DURATION = TimeUnit.NANOSECONDS.convert(30, TimeUnit.SECONDS);
    private static final int THREADS_NUMBER = 2;
    private static final ThreadLocal<AtomicLong> COUNTER = new ThreadLocal<AtomicLong>() {
        @Override
        protected AtomicLong initialValue() {
            return new AtomicLong();
        }
    };
    private static final ThreadLocal<AtomicLong> DUMMY_DATA = new ThreadLocal<AtomicLong>() {
        @Override
        protected AtomicLong initialValue() {
            return new AtomicLong();
        }
    };
    private static final AtomicLong DUMMY_COUNTER = new AtomicLong();
    private static final AtomicLong END_TIME = new AtomicLong(System.nanoTime() + DURATION);

    private static final List<ThreadLocal<CharSequence>> DUMMY_SOURCE = new ArrayList<ThreadLocal<CharSequence>>();
    static {
        for (int i = 0; i < 40; ++i) {
            DUMMY_SOURCE.add(new ThreadLocal<CharSequence>());
        }
    }

    private static final Set<Long> COUNTERS = new ConcurrentSkipListSet<Long>();

    public static void main(String[] args) throws Exception {
        final CountDownLatch startLatch = new CountDownLatch(THREADS_NUMBER);
        final CountDownLatch endLatch = new CountDownLatch(THREADS_NUMBER);

        for (int i = 0; i < THREADS_NUMBER; i++) {
            new Thread() {
                @Override
                public void run() {
                    initDummyData();
                    startLatch.countDown();
                    try {
                        startLatch.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    while (System.nanoTime() < END_TIME.get()) {
                        doJob();
                    }
                    COUNTERS.add(COUNTER.get().get());
                    DUMMY_COUNTER.addAndGet(DUMMY_DATA.get().get());
                    endLatch.countDown();
                }
            }.start();
        }
        startLatch.await();
        END_TIME.set(System.nanoTime() + DURATION);

        endLatch.await();
        printStatistics();
    }

    private static void initDummyData() {
        for (ThreadLocal<CharSequence> threadLocal : DUMMY_SOURCE) {
            threadLocal.set(getRandomString());
        }
    }

    private static CharSequence getRandomString() {
        StringBuilder result = new StringBuilder();
        Random random = new Random();
        for (int i = 0; i < 127; ++i) {
            result.append((char)random.nextInt(0xFF));
        }
        return result;
    }

    private static void doJob() {
        Random random = new Random();
        for (ThreadLocal<CharSequence> threadLocal : DUMMY_SOURCE) {
            for (int i = 0; i < threadLocal.get().length(); ++i) {
                DUMMY_DATA.get().addAndGet(threadLocal.get().charAt(i) << random.nextInt(31));
            }
        }
        COUNTER.get().incrementAndGet();
    }

    private static void printStatistics() {
        long total = 0L;
        for (Long counter : COUNTERS) {
            total += counter;
        }
        System.out.printf("Total iterations number: %d, dummy data: %d, distribution:%n", total, DUMMY_COUNTER.get());
        for (Long counter : COUNTERS) {
            System.out.printf("%f%%%n", counter * 100d / total);
        }
    }
}

我对两个线程和十个线程的场景进行了四次测试，结果表明性能损失约为 2.5%(两个线程迭代 78626 次，十个线程迭代 76754 次)，线程使用的系统资源大致相等。

此外，“java.util.concurrent” 作者假设上下文切换时间约为 2000-4000 个 CPU 周期:

public class Exchanger<V> {
   ...
   private static final int NCPU = Runtime.getRuntime().availableProcessors();
   ....
   /**
    * The number of times to spin (doing nothing except polling a
    * memory location) before blocking or giving up while waiting to
    * be fulfilled.  Should be zero on uniprocessors.  On
    * multiprocessors, this value should be large enough so that two
    * threads exchanging items as fast as possible block only when
    * one of them is stalled (due to GC or preemption), but not much
    * longer, to avoid wasting CPU resources.  Seen differently, this
    * value is a little over half the number of cycles of an average
    * context switch time on most systems.  The value here is
    * approximately the average of those across a range of tested
    * systems.
    */
   private static final int SPINS = (NCPU == 1) ? 0 : 2000;