Re: PREEMPT_RT and I-PIPE: the numbers, take 3

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@us.ibm.com> wrote:

> > another point is that this test is measuring the overhead of PREEMPT_RT, 
> > without measuring the benefit of the cost: RT-task scheduling latencies.  
> > We know since the rtirq patch (to which i-pipe is quite similar) that we 
> > can achieve good irq-service latencies via relatively simple means, but 
> > that's not what PREEMPT_RT attempts to do. (PREEMPT_RT necessarily has 
> > to have good irq-response times too, but much of the focus went to the 
> > other aspects of RT task scheduling.)
> 
> Agreed, a PREEMPT_RT-to-IPIPE comparison will never be an 
> apples-to-apples comparison.  Raw data will never be a substitute for 
> careful thought, right?  ;-)

well, it could still be tested, since it's so easy: the dohell script is 
already doing all of that as it runs rtc_wakeup - which runs a 
SCHED_FIFO task and carefully measures wakeup latencies. If it is used 
with 1024 Hz (the default) and it can be used in every test without 
impacting the system load in any noticeable way.

	Ingo