Re: 2.6.12-rc6-mm1

[Con Kolivas]

On Sun, 12 Jun 2005 09:27, Martin J. Bligh wrote:
> >> not sure what the benefits of the patch are, 

I should have answered this. Since we moved to one runqueue per cpu with the 
current scheduler, 'nice' levels basically fall apart on SMP. Balancing tends 
to group together all the wrong tasks to have any meaningful 'nice' support 
where often on a 2 cpu machine if we run 4 tasks, 2 nice 0 and 2 nice 19 we 
end up with:

cpu 1: nice 19 + nice 19
cpu 2: nice 0 + nice 0

which means each nice 19 task gets half a cpu and each nice 0 task gets half a 
cpu which is lousy fairness. 

The smp nice patches should end up with
cpu 1: nice 0 + nice 19
cpu 2: nice 0 + nice 19

so that the nice 0 tasks get 95% of a cpu and nice 19 tasks get 5% of a cpu.

The patches should balance things as fairly as possible according to nice 
levels across cpus. As you can see this is clearly a bug in behaviour and has 
been a showstopper for many trying to move from 2.4.

Cheers,
Con

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