cov/cor中有遗漏值

在这篇文章中，我演示了许多想法和技术：

1. 如何编写一个简单的非阻塞NIO客户端/服务器
2. 协同遗漏的影响
3. 如何测量百分位数的延迟（相对于简单平均）
4. 如何在计算机上计时延迟回送

我最近正在为客户端服务器应用程序开发低延迟基准。 最初，我是在使用环回TCP的单台计算机上模拟基准测试。 我要量化的第一个指标是允许简单的环回延迟需要多少记录的延迟。 这样，我便可以更清楚地了解实际应用程序增加的延迟。

为此，我创建了一个程序（文章末尾的代码），该程序将一个字节从客户端传输到服务器，然后再传输回来。 重复执行此操作并处理结果。

该程序是使用非阻塞Java NIO编写的，以尽可能优化环回延迟。

比记录平均时间更重要的是，记录了百分比延迟。 （有关如何测量延迟的讨论，请参见此处的上一篇文章）。 至关重要的是，协调遗漏的代码因素。 （要了解更多关于此看到这里从吉尔·特内）。 简而言之，您不必从开始就开始计时，而应该从开始就开始计时。

这些是我2岁的MBP的结果。

Starting latency test rate: 80000
Average time 2513852
Loop back echo latency was 2514247.3/3887258.6 4,196,487/4,226,913 4,229,987/4230294 4,230,294 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 70000
Average time 2327041
Loop back echo latency was 2339701.6/3666542.5 3,957,860/3,986,626 3,989,404/3989763 3,989,763 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 50000
Average time 1883303
Loop back echo latency was 1881621.0/2960104.0 3,203,771/3,229,260 3,231,809/3232046 3,232,046 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 30000
Average time 1021576
Loop back echo latency was 1029566.5/1599881.0 1,726,326/1,739,626 1,741,098/1741233 1,741,233 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 20000
Average time 304
Loop back echo latency was 65.6/831.2 3,632/4,559 4,690/4698 4,698 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 10000
Average time 50
Loop back echo latency was 47.8/57.9 89/120 152/182 182 us for 50/90 99/99.9 99.99/99.999 worst %tile

将这些结果与我没有纠正遗漏的情况进行比较：

Starting latency test rate: 80000
Average time 45
Loop back echo latency was 44.1/48.8 71/105 124/374 374 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 70000
Average time 45
Loop back echo latency was 44.1/48.9 76/106 145/358 358 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 50000
Average time 45
Loop back echo latency was 43.9/48.8 74/105 123/162 162 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 30000
Average time 45
Loop back echo latency was 44.0/48.8 73/104 129/147 147 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 20000
Average time 45
Loop back echo latency was 44.7/49.6 78/107 135/311 311 us for 50/90 99/99.9 99.99/99.999 worst %tile
Starting latency test rate: 10000
Average time 46
Loop back echo latency was 45.1/50.8 81/112 144/184 184 us for 50/90 99/99.9 99.99/99.999 worst %tile

如您所见，吞吐量的影响被完全忽略了！ 看起来，即使以每秒80,000条消息的速度运行，您的99.99个百分位数仍为374us，而实际上却远大于此。

实际上，只有在吞吐量接近每秒10,000时，您才能达到目标延迟。 如您所直观地理解的那样，在吞吐量和延迟之间需要权衡取舍。

此测试的代码如下：

package util;import java.io.EOFException;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.nio.ByteBuffer;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Arrays;/*** Created by daniel on 02/07/2015.* Simple program to test loopback speeds and latencies.*/
public class LoopBackPingPong {public final static int PORT = 8007;public void runServer(int port) throws IOException {ServerSocketChannel ssc = ServerSocketChannel.open();ssc.bind(new InetSocketAddress(port));System.out.println("listening on " + ssc);final SocketChannel socket = ssc.accept();socket.socket().setTcpNoDelay(true);socket.configureBlocking(false);new Thread(() -> {long totalTime = 0;int count = 0;try {System.out.println("Connected " + socket);ByteBuffer bb = ByteBuffer.allocateDirect(1);int length;while ((length = socket.read(bb)) >= 0) {if (length > 0) {long time = System.nanoTime();bb.flip();bb.position(0);count++;if (socket.write(bb) < 0)throw new EOFException();bb.clear();totalTime += System.nanoTime() - time;}}} catch (IOException ignored) {} finally {System.out.println("Total server time " + (totalTime / count) / 1000);System.out.println("... disconnected " + socket);try {socket.close();} catch (IOException ignored) {}}}).start();}public void testLatency(int targetThroughput, SocketChannel socket) throws IOException {System.out.println("Starting latency test rate: " + targetThroughput);int tests = Math.min(18 * targetThroughput, 100_000);long[] times = new long[tests];int count = 0;long now = System.nanoTime();long rate = (long) (1e9 / targetThroughput);ByteBuffer bb = ByteBuffer.allocateDirect(4);bb.putInt(0, 0x12345678);for (int i = -20000; i < tests; i++) {//now += rate;//while (System.nanoTime() < now)//    ;now = System.nanoTime();bb.position(0);while (bb.remaining() > 0)if (socket.write(bb) < 0)throw new EOFException();bb.position(0);while (bb.remaining() > 0)if (socket.read(bb) < 0)throw new EOFException();if (bb.getInt(0) != 0x12345678)throw new AssertionError("read error");if (i >= 0)times[count++] = System.nanoTime() - now;}System.out.println("Average time " + (Arrays.stream(times).sum() / times.length) / 1000);Arrays.sort(times);System.out.printf("Loop back echo latency was %.1f/%.1f %,d/%,d %,d/%d %,d us for 50/90 99/99.9 99.99/99.999 worst %%tile%n",times[times.length / 2] / 1e3,times[times.length * 9 / 10] / 1e3,times[times.length - times.length / 100] / 1000,times[times.length - times.length / 1000] / 1000,times[times.length - times.length / 10000] / 1000,times[times.length - times.length / 100000] / 1000,times[times.length - 1] / 1000);}public static void main(String... args) throws Exception {int port = args.length < 1 ? PORT : Integer.parseInt(args[0]);LoopBackPingPong lbpp = new LoopBackPingPong();new Thread(() -> {try {lbpp.runServer(port);} catch (IOException e) {Jvm.rethrow(e);}}).start();//give the server a chance to startThread.sleep(1000);SocketChannel socket = SocketChannel.open(new InetSocketAddress("localhost", port));socket.socket().setTcpNoDelay(true);socket.configureBlocking(false);for (int i : new int[]{80_000, 70_000, 50_000, 30_000, 20_000, 10_000})lbpp.testLatency(i, socket);System.exit(0);}}

翻译自: https://www.javacodegeeks.com/2015/07/the-effect-of-co-ordinated-omission-measure-loopback-latency-using-a-simple-nio-clientserver.html

cov/cor中有遗漏值