nice() is not nice to my app..

[thehadar]

Hey guys,
I'm working on an Embedded Linux kernel 2.6.36.4.
This machine runs a main utility that interacts with the kernel. Let's call this utility MainUtility.
I wrote a TestUtilty that runs for 5 seconds and increments a variable X in a loop. After 5 seconds the TestUtility displays X's value and dye.
I execute the TestUtility with different nice values while the MainUtility is running and while the MainUtility is not running.
I do not understand my findings. I hope someone here could shed some light on the matter.
(*) Please note, I did every test 100 times, and the result was consistent.

A) When the MainUtility is not running if I execute the TestUtility with nice=0,nice=-20 and nice=20 I get the following result:
nice = 0 after 5 seconds X~=1,553,500
nice = -20 after 5 seconds X~=1,345,000
nice = 20 after 5 seconds X~=1,553,500

It's strange. Shouldn't nice=-20 gets the best results?
Can someone explain why did the opposite occur?

B) When I execute the TestUtility while the MainUtility is running, with nice=0,nice=-20 and nice=20 I get the following result:
nice = 0 after 5 seconds X~=1,000,000
nice = -20 after 5 seconds X~=1,000,000
nice = 20 after 5 seconds X~=1,000,000

Can someone explain how come the nice has no effect??

Thanks from advance!
Hadar

[Rubberman]

It depends upon the scheduler the kernel is using. A lot of embedded systems use the "real-time" scheduler, so I'm not sure what effect that nice has upon them. In any case, nice with a large negative number (like -20) will only suggest to the kernel that your application gets priority scheduling when other applications need to run that have a higher niceness factor. It still doesn't affect I/O and other kernel operations.

If you need deterministic performance behavior, and absolute priority enforcement (and no I/O priority inversions), then you will need to move to a true real-time operating system such as QNX.