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

[Gregory Haskins]

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


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