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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