Re: Interbench real time benchmark results

[Ingo Molnar <mingo@elte.hu>]

* Con Kolivas <kernel@kolivas.org> wrote:

> Not entirely what some would expect. Very little difference under low 
> loads, but the maximum latencies exhibited are about the same at 
> 300us. However they hare under different workloads. With these 
> worklods, on this hardware, running these real time simulations there 
> is not a convincing argument for CONFIG-PREEMPT. Note that running 
> interbench with the non-real time benchmarks also does not show a 
> convincing reason for preempt.

while i do like the PREEMPT_RT results, i think we need to do two more 
things to have total confidence in the numbers:

 - i think we'll need to increase the number of sample points, by both
   increasing the frequency of samples, and by lengthening the
   test-time - even if just for a single testrun. Some of the worst-case 
   latencies i care about in PREEMPT_RT trigger only once every couple 
   of million interrupts (!). For human interactivity we probably dont 
   care that much though.

 - many of the worst-case latencies relate to some sort of extreme 
   situation within a particular algorithm. E.g. lots of tasks being 
   around. Do this for example:

	hackbench 50

   and Ctrl-Z it after a couple of seconds. You'll see a 1msec (or 
   larger) blip.

   or, fill up swapspace, so that the swap allocation map gets filled
   up.

 - networking is another frequent source of latencies - it might make 
   sense to add a workload doing lots of socket IO. (localhost might be 
   enough, but not for everything)

	Ingo