Re: [PATCH] cpu scheduler: unsquish dynamic priorities

[Ingo Molnar <mingo@elte.hu>]

* Peter Williams <pwil3058@bigpond.net.au> wrote:

> The problem:
> 
> The current scheduler implementation maps 40 nice values and 10 bonus 
> values into only 40 priority slots on the run queues.  This results in 
> the dynamic priorities of tasks at either end of the nice scale being 
> squished up.  E.g. all tasks with nice in the range -20 to -16 and the 
> maximum of 10 bonus points will end up with a dynamic priority of 
> MAX_RT_PRIO and all tasks with nice in the range 15 to 19 and no bonus 
> points will end up with a dynamic priority of MAX_PRIO - 1.
> 
> Although the fact that niceness is primarily implemented by time slice 
> size means that this will have little or no adverse effect on the long 
> term allocation of CPU resources due to niceness, it could adversely 
> effect latency as it will interfere with preemption.

this property of the priority distribution was intentional from me, i 
wanted to have an easy way to test 'no priority boosting downwards' 
(nice +19) and 'no priority boosting upwards' (nice -20) conditions. But 
i like your patch, because it simplifies effective_prio() a bit, and 
with SCHED_BATCH we'll have the 'no boosting' property anyway. Could you 
redo the patch against the current scheduler queue in -mm, so that we 
can try it out in -mm?

Btw., another user-visible effect is that task_prio() will return the 
new range, which will be visible in e.g. 'top'. I dont think it will be 
confusing though.

	Ingo