Re: Ten percent test

[Mike Galbraith]

On Mon, 2007-04-09 at 07:38 +0200, Mike Galbraith wrote:

> I don't think you can have very much effect on latency using nice with
> SD once the CPU is fully utilized.  See below.
> 
> /*
>  * This contains a bitmap for each dynamic priority level with empty slots
>  * for the valid priorities each different nice level can have. It allows
>  * us to stagger the slots where differing priorities run in a way that
>  * keeps latency differences between different nice levels at a minimum.
>  * ie, where 0 means a slot for that priority, priority running from left to
>  * right:
>  * nice -20 0000000000000000000000000000000000000000
>  * nice -10 1001000100100010001001000100010010001000
>  * nice   0 0101010101010101010101010101010101010101
>  * nice   5 1101011010110101101011010110101101011011
>  * nice  10 0110111011011101110110111011101101110111
>  * nice  15 0111110111111011111101111101111110111111
>  * nice  19 1111111111111111111011111111111111111111
>  */
> 
> Nice allocates bandwidth, but as long as the CPU is busy, tasks always
> proceed downward in priority until they hit the expired array.  That's
> the design.

There's another aspect of this that may require some thought - kernel
threads.  As load increases, so does rotation length.  Would you really
want CPU hogs routinely preempting house-keepers under load?

	-Mike

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