简单循环的速度 [英] Speed of simple loop

问题描述

为了测试速度，我在 Java 中做了一个简单的循环.与 C 中的相同循环相比，它需要更多时间.执行 20 亿次迭代大约需要 6.5 秒

I have made a simple loop in Java just to test the speed. Compared to the same loop in C it takes a lot more time. 2 billion iterations takes about 6.5 seconds when its executed

如果它被认为很慢 - 可以做些什么来提高性能?

If its considered to be slow - what could one do to improve the performance?

能不能怪JVM的启动?或者 - JIT 编译器没有完成它的工作吗?

Could one blame the startup of the JVM? Or - is the JIT-compiler not doing its job?

• 平台:windows xp

• 处理器速度:3.4 GHz

• platform: windows xp

• processor speed: 3.4 GHz

public class Jrand {

public static void main (String[] args) {

  float f;

  long startTime = System.currentTimeMillis();

  for (int i = 0; i < 2000000000; i++) {
    f = i * 0.0001F;
  }
  long endTime = System.currentTimeMillis();
  float totalTime = (endTime - startTime);
  System.out.println("time: " + totalTime/1000);
  }
}

推荐答案

这个

for (int j = 0; j < 10; j++) {
    float f;

    long start = System.nanoTime();

    for (int i = 0; i < 2000000000; i++) {
        f = i * 0.0001F;
    }
    long end = System.nanoTime();
    long timeToCallNanoTime = System.nanoTime() - end;
    long time = Math.max(0, end - start - timeToCallNanoTime);
    System.out.println("time: " + time + " ns.");
}

印刷品

time: 2580790 ns.
time: 4241443 ns.
time: 17 ns.
time: 0 ns.
time: 0 ns.
time: 0 ns.
time: 0 ns.
time: 0 ns.
time: 5 ns.
time: 0 ns.

如您所见，JIT 已经完全优化了循环.没有比这更快的了.

As you can see the JIT has optimised the loop away completely. It can't get faster than that.

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