Re: [PATCH v3 17/17] (Avoid overload)

Gregory Haskins
2007-11-17T17:46:56Z

>>> On Sat, Nov 17, 2007 at 1:33 AM, in message
<20071117063318.GA31442@goodmis.org>, Steven Rostedt <rostedt@goodmis.org>
wrote:

> Sorry! I forgot to put in a prologue for this patch.
>
> Here it is.
>
> ====
>
> This patch changes the searching for a run queue by a waking RT task
> to try to pick another runqueue if the currently running task
> is an RT task.
>
> The reason is that RT tasks behave different than normal
> tasks. Preempting a normal task to run a RT task to keep
> its cache hot is fine, because the preempted non-RT task
> may wait on that same runqueue to run again unless the
> migration thread comes along and pulls it off.
>
> RT tasks behave differently. If one is preempted, it makes
> an active effort to continue to run. So by having a high
> priority task preempt a lower priority RT task, that lower
> RT task will then quickly try to run on another runqueue.
> This will cause that lower RT task to replace its nice
> hot cache (and TLB) with a completely cold one. This is
> for the hope that the new high priority RT task will keep
> its cache hot.
>
> Remeber that this high priority RT task was just woken up.
> So it may likely have been sleeping for several milliseconds,
> and will end up with a cold cache anyway. RT tasks run till
> they voluntarily stop, or are preempted by a higher priority
> task. This means that it is unlikely that the woken RT task
> will have a hot cache to wake up to. So pushing off a lower
> RT task is just killing its cache for no good reason.

You make some excellent points here. Out of curiosity, have you tried a comparison to see if it helps?

-Greg