Why does “while(true)” without “Thread.sleep” cause 100% CPU usage on Linux but not on Windows?

Question

I have created a simple program in java:

public static void main(String[] args) throws InterruptedException {

    while (true) 

        ;

}

If I run this on a Linux machine, it shows 100% CPU usage, but doesn't cause the OS to appear slow. However, if I run the exact same code on Windows, it only shows about 20% CPU usage.

I am using Oracle JRE on Windows and OpenJDK 6 on Linux.

I'm wondering if Windows' scheduler preempt threads randomly and Linux's doesn't?

Answer 1

By default, top on Linux runs in so-called IRIX mode, while the Windows Task Manager does not. Let's say you have 4 cores:

• With IRIX mode on, 1 fully utilized core is 100% and 4 cores are 400%.
• With IRIX mode off, 1 fully utilized core is 25% and 4 cores are 100%.

This means that by default, top on Linux will show an infinite loop as ~100% and Windows will show it as ~25%, and it means exactly the same thing.

You can toggle IRIX mode while top is running with Shift+i. This will make the numbers match up.