cov/cor中有遺漏值

在這篇文章中，我演示了許多想法和技術：

如何編寫一個簡單的非阻塞NIO客戶端/服務器

協同遺漏的影響

如何測量百分位數的延遲（相對于簡單平均）

如何在計算機上計時延遲回送

我最近正在為客戶端服務器應用程序開發低延遲基準。 最初，我是在使用環回TCP的單臺計算機上模擬基準測試。 我要量化的第一個指標是允許簡單的環回延遲需要多少記錄的延遲。 這樣，我便可以更清楚地了解實際應用程序增加的延遲。

為此，我創建了一個程序（文章末尾的代碼），該程序將一個字節從客戶端傳輸到服務器，然后再傳輸回來。 重復執行此操作并處理結果。

該程序是使用非阻塞Java NIO編寫的，以盡可能優化環回延遲。

比記錄平均時間更重要的是，記錄了百分比延遲。 （有關如何測量延遲的討論，請參見此處的上一篇文章）。 至關重要的是，協調遺漏的代碼因素。 （要了解更多關于此看到這里從吉爾·特內）。 簡而言之，您不必從開始就開始計時，而應該從開始就開始計時。

這些是我2歲的MBP的結果。

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

將這些結果與我沒有糾正遺漏的情況進行比較：

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

如您所見，吞吐量的影響被完全忽略了！ 看起來，即使以每秒80,000條消息的速度運行，您的99.99個百分位數仍為374us，而實際上卻遠大于此。

實際上，只有在吞吐量接近每秒10,000時，您才能達到目標延遲。 如您所直觀地理解的那樣，在吞吐量和延遲之間需要權衡取舍。

此測試的代碼如下：

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);}}

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

cov/cor中有遺漏值

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