Re: [PATCH] V-3.0 Single Priority Array O(1) CPU Scheduler Evaluation

Peter Williams
2004-08-04T01:37:10Z

> William Lee Irwin III wrote:
>
>>>In such schemes, realtime tasks are considered separately from
>>>timesharing tasks. Finding a task to run or migrate proceeds with a
>>>circular search of the portion of the bitmap used for timesharing tasks
>>>after a linear search of that for RT tasks. The list to enqueue a
>>>timesharing task in is just an offset from the fencepost determined by
>>>priority. Dequeueing is supported with a tag for actual array position.
>>>I did this for aperiodic queue rotations, which differs from your SPA.
>
>
> On Wed, Aug 04, 2004 at 10:37:57AM +1000, Peter Williams wrote:
>
>>While pondering this I have stumbled on a problem that rules out using a
>>rotating list for implementing promotion. The problem is that one of
>>the requirements is that once a SCHED_NORMAL task is promoted to the
>>MAX_RT_PRIO slot it stays there (as far as promotion is concerned).
>>With the rotating list this isn't guaranteed and, in fact, any tasks
>>that are in the MAX_RT_PRIO slot when promotion occurs will actually be
>>demoted to IDLE_PRIO - 1.
>
>
> Aperiodic rotations defer movement until MAX_RT_PRIO's slot is evacuated.

Unfortunately, to ensure no starvation, promotion has to continue even
when there are tasks in MAX_RT_PRIO's slot.

>
>
> On Wed, Aug 04, 2004 at 10:37:57AM +1000, Peter Williams wrote:
>
>>Promotion should be a rare event as it is unnecessary if there's less
>>than two tasks on the runqueue and when there are more than one task on
>>the runqueue the interval between promotions increases linearly with the
>>number of runnable tasks. It is also an O(1) operation albeit with a
>>constant factor determined by the number of occupied SCHED_NORMAL
>>priority slots.
>
>
> The asymptotics were in terms of SCHED_NORMAL priorities.
>
>
> On Wed, Aug 04, 2004 at 10:37:57AM +1000, Peter Williams wrote:
>
>>I will modify the code to take better advantage of the fact that
>>promotion is not required when the number of runnable tasks is less than
>>2 e.g. by resetting next_prom_due so that the first promotion after the
>>number of runnable tasks exceeds 1 will only occur after a full
>>promotion interval has expired. At normal loads (and with sensible
>>promotion interval settings i.e. greater than the time slice size) this
>>should result in promotion never (or hardly ever) occurring and the
>>overhead of do_promotions() will only have to be endured when it's
>>absolutely necessary.
>
>
> The primary concern was that ticklessness etc. may require it to occur
> during context switches.

On a tickless system, I'd consider using a timer to control when
do_promotions() gets called. I imagine something similar will be
necessary to manage time slices?