why does Virtualbox use 15-20% CPU when VM is paused?

In order to lessen VirtualBox CPU usage at all times, resort to this weird hack.

Create a new virtual machine and do not install an operating system to it. Tell VirtualBox it will run DOS and give it the absolute minimum resources. Do not install an O/S. Run it, let it error out on boot and minimize it.

While running your real O/S in a second VM, you will see your idle Virtualbox CPU utilization drop to 3-5%.

Idea from jed4czar: http://ubuntuforums.org/showthread.php?s=58e862a814e65eb96f8fe8389b615366&t=838073&page=2

EDIT: To answer your questions directly

why does Virtualbox use 15-20% CPU when VM is paused?

It is a bug. It is always using 15-20% more CPU than it needs to, when any one VM is up, unless the hack provided is used.

Is there a way to avoid it?

See hack above.

I tried the hack described above with the DOS VM, but with no success (running Ubuntu 12.04 guests on a MacBook Pro with OS X). I also tried the kernel parameter changes mentioned in the Oracle thread, again no change. No matter what I did, my guests seemed to eat 15-20% CPU each. However, I noticed that the one guest I happened to assign 2 CPUs was not chewing 15-20% - it was behaving as expected.

Sure enough, when I switched the others to 2 CPU's the issue disappeared. From previous experience, I know that switching to 2 CPUs also enables the IO APIC option in the motherboard section so I suspected that was the truly interesting change. That is, this one:

Note that you have to have the machine shut down to change this setting, otherwise it is greyed out. Once I had enabled this on the guests and restart, it didn't matter if I had 1 or more CPUs, the 15-20% CPU utilization went away, so I thought I would share my solution here.