Re: [RFC 2/5] sched: Add CPU rate soft caps

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Balbir Singh wrote:
On 5/28/06, Peter Williams <[email protected]> wrote:
Peter Williams wrote:
> Balbir Singh wrote:
>> On 5/26/06, Peter Williams <[email protected]> wrote:
>> <snip>
>>> Notes:
>>> 1. To minimize the overhead incurred when testing to skip caps
>>> processing for
>>> uncapped tasks a new flag PF_HAS_CAP has been added to flags.
>>> 2. The implementation involves the addition of two priority slots to the
>>> run queue priority arrays and this means that MAX_PRIO no longer
>>> represents the scheduling priority of the idle process and can't be
>>> used to
>>> test whether priority values are in the valid range. To alleviate this
>>> problem a new function sched_idle_prio() has been provided.
>> I am a little confused by this. Why link the bandwidth expired tasks a
>> cpu (its caps) to a priority slot? Is this a hack to conitnue using
>> the prio_array? why not move such tasks to the expired array?
> Because it won't work as after the array switch they may get to run
> before tasks who aren't exceeding their cap (or don't have a cap).

That behaviour would be fair.

Caps aren't about being fair. In fact, giving a task a cap is an explicit instruction to the scheduler that the task should be treated unfairly in some circumstances (namely when it's exceeding that cap).

Similarly, the interactive bonus mechanism is not about fairness either. It's about giving tasks that are thought to be interactive an unfair advantage so that the user experiences good responsiveness.

Peter Williams                                   [email protected]

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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