>>> On Sat, Nov 17, 2007 at 1:33 AM, in message <[EMAIL PROTECTED]>, Steven Rostedt <[EMAIL PROTECTED]> 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
