根据其文档, System.nanoTime 从某个固定但任意的原始时间返回纳秒。但是,在所有 x64 机器上,我尝试了下面的代码,出现了时间跳跃,移动了固定的原始时间。我使用替代方法(此处为 currentTimeMillis)获取正确时间的方法可能存在一些缺陷。然而,测量相对时间(持续时间)的主要目的也受到负面影响。
当我将不同的队列与 LMAX 的 Disruptor 进行比较时,我在尝试测量延迟时遇到了这个问题,有时我会得到非常负的延迟。在这些情况下,开始和结束时间戳是由不同的线程创建的,但延迟是在这些线程完成后计算的。
我的代码在这里使用 nanoTime 花费时间,以 currentTimeMillis 时间计算固定原点,并在调用之间比较该原点。既然我必须在这里问一个问题:这段代码有什么问题?为什么它观察到违反固定来源合同的行为?还是没有?
import java.text.*;
/**
* test coherency between {@link System#currentTimeMillis()} and {@link System#nanoTime()}
*/
public class TimeCoherencyTest {
static final int MAX_THREADS = Math.max( 1, Runtime.getRuntime().availableProcessors() - 1);
static final long RUNTIME_NS = 1000000000L * 100;
static final long BIG_OFFSET_MS = 2;
static long startNanos;
static long firstNanoOrigin;
static {
initNanos();
}
private static void initNanos() {
long millisBefore = System.currentTimeMillis();
long millisAfter;
do {
startNanos = System.nanoTime();
millisAfter = System.currentTimeMillis();
} while ( millisAfter != millisBefore);
firstNanoOrigin = ( long) ( millisAfter - ( startNanos / 1e6));
}
static NumberFormat lnf = DecimalFormat.getNumberInstance();
static {
lnf.setMaximumFractionDigits( 3);
lnf.setGroupingUsed( true);
};
static class TimeCoherency {
long firstOrigin;
long lastOrigin;
long numMismatchToLast = 0;
long numMismatchToFirst = 0;
long numMismatchToFirstBig = 0;
long numChecks = 0;
public TimeCoherency( long firstNanoOrigin) {
firstOrigin = firstNanoOrigin;
lastOrigin = firstOrigin;
}
}
public static void main( String[] args) {
Thread[] threads = new Thread[ MAX_THREADS];
for ( int i = 0; i < MAX_THREADS; i++) {
final int fi = i;
final TimeCoherency tc = new TimeCoherency( firstNanoOrigin);
threads[ i] = new Thread() {
@Override
public void run() {
long start = getNow( tc);
long firstOrigin = tc.lastOrigin; // get the first origin for this thread
System.out.println( "Thread " + fi + " started at " + lnf.format( start) + " ns");
long nruns = 0;
while ( getNow( tc) < RUNTIME_NS) {
nruns++;
}
final long runTimeNS = getNow( tc) - start;
final long originDrift = tc.lastOrigin - firstOrigin;
nruns += 3; // account for start and end call and the one that ends the loop
final long skipped = nruns - tc.numChecks;
System.out.println( "Thread " + fi + " finished after " + lnf.format( nruns) + " runs in " + lnf.format( runTimeNS) + " ns (" + lnf.format( ( double) runTimeNS / nruns) + " ns/call) with"
+ "\n\t" + lnf.format( tc.numMismatchToFirst) + " different from first origin (" + lnf.format( 100.0 * tc.numMismatchToFirst / nruns) + "%)"
+ "\n\t" + lnf.format( tc.numMismatchToLast) + " jumps from last origin (" + lnf.format( 100.0 * tc.numMismatchToLast / nruns) + "%)"
+ "\n\t" + lnf.format( tc.numMismatchToFirstBig) + " different from first origin by more than " + BIG_OFFSET_MS + " ms"
+ " (" + lnf.format( 100.0 * tc.numMismatchToFirstBig / nruns) + "%)"
+ "\n\t" + "total drift: " + lnf.format( originDrift) + " ms, " + lnf.format( skipped) + " skipped (" + lnf.format( 100.0 * skipped / nruns) + " %)");
}};
threads[ i].start();
}
try {
for ( Thread thread : threads) {
thread.join();
}
} catch ( InterruptedException ie) {};
}
public static long getNow( TimeCoherency coherency) {
long millisBefore = System.currentTimeMillis();
long now = System.nanoTime();
if ( coherency != null) {
checkOffset( now, millisBefore, coherency);
}
return now - startNanos;
}
private static void checkOffset( long nanoTime, long millisBefore, TimeCoherency tc) {
long millisAfter = System.currentTimeMillis();
if ( millisBefore != millisAfter) {
// disregard since thread may have slept between calls
return;
}
tc.numChecks++;
long nanoMillis = ( long) ( nanoTime / 1e6);
long nanoOrigin = millisAfter - nanoMillis;
long oldOrigin = tc.lastOrigin;
if ( oldOrigin != nanoOrigin) {
tc.lastOrigin = nanoOrigin;
tc.numMismatchToLast++;
}
if ( tc.firstOrigin != nanoOrigin) {
tc.numMismatchToFirst++;
}
if ( Math.abs( tc.firstOrigin - nanoOrigin) > BIG_OFFSET_MS) {
tc.numMismatchToFirstBig ++;
}
}
}
现在我做了一些小改动。基本上,我将两个 currentTimeMillis 调用之间的 nanoTime 调用括起来,以查看线程是否已被重新调度(这应该比 currentTimeMillis 分辨率多)。在这种情况下,我忽略了循环周期。实际上,如果我们知道 nanoTime 足够快(如在 Ivy Bridge 等较新的架构上),我们可以将 currentTimeMillis 与 nanoTime 括起来。
现在超过 10 毫秒的长跳跃消失了。相反,我们计算每个线程距离第一个原点超过 2 毫秒的时间。在我测试过的机器上,对于 100 秒的运行时间,调用之间总是有接近 200.000 次跳转。对于那些我认为 currentTimeMillis 或 nanoTime 可能不准确的情况。