Re: RT task scheduling

[Vernon Mauery <vernux@us.ibm.com>]

On Thursday 06 April 2006 15:35, you wrote:
> On Thursday 06 April 2006 11:16, you wrote:
> > On Thursday 06 April 2006 00:37, Ingo Molnar wrote:
> > > * Darren Hart <darren@dvhart.com> wrote:

[snip]

> On a related note, I tried observing the rt stats in /proc/stats while
> running sched_football on 2.6.16-rt13.  The entire log is nearly a MB so I
> placed it on my website for reference
> (http://www.dvhart.com/~dvhart/sched_football_stats.log), an excerpt
> follows:
>
> ---------------------
>
> The following is the output of sched_football run with 1 thread for offense
> and 1 thread for defense on a 4 way machine.  The ball position is
> irrelevant in this case since there are more CPUs than threads (they should
> all be able to run).  What is disturbing is that over the entire run, I
> never see RT tasks on every CPU.  Even though there are usually 5 total
> runnnable threads, we constantly see groupings of 2 and 3 on the runqueues
> while the others have no running rt tasks.
>
> Looking back, I should have added a sleep to the loop - oops - still, I
> think the data is interesting and suggests a problem with sceduling RT
> tasks across all available CPUs.  Does this seem like a valid test to
> everyone?  Is there perhaps some explanation as to why this would be
> expected (when the cat process get's to read the proc information or
> something) ?

A better way to do the logging loop would be to write a program that simply 
reads the contents of /proc/stat, filters for rt and writes out to your log 
and run that at a high realtime priority.  That way, the logging task will be 
(more) sure to be capturing the data for the other 3 CPUs constantly and get 
a clearer picture of what is happening.  Naturally, the monitoring task would 
interfere with the execution of the rest of the football game, but it should 
just hog the single CPU while the rest of the game could play out on the 
remaining three.  And from the logs below, it seemed to only use two CPUs 
part of the time anyway.

Having a constantly running logger would also avoid all the forky-execy the 
bash loop has.

--Vernon

> # ./sched_football 1 60
> Starting 1 offense threads at priority 15
> Starting 1 defense threads at priority 30
> Starting referee thread
> Game On (60 seconds)!
> Game Over!
> Final ball position: 20359767
>
> # while true; do clear; cat /proc/stat | grep rt >>
> sched_football_stats.log; done
>
> sched_football_stats.log
> ------------------------------
> rt_overload_schedule: 57768
> rt_overload_wakeup:   157501
> rt_overload_pulled:   13722934
> rt_nr_running(0): 0
> rt_nr_running(1): 0
> rt_nr_running(2): 0
> rt_nr_running(3): 0
> rt_overload: 0
> rt_overload_schedule: 57769
> rt_overload_wakeup:   157514
> rt_overload_pulled:   13722937
> rt_nr_running(0): 0
> rt_nr_running(1): 2
> rt_nr_running(2): 3
> rt_nr_running(3): 0
> rt_overload: 2
> ...
> rt_overload_schedule: 57774
> rt_overload_wakeup:   157738
> rt_overload_pulled:   13722941
> rt_nr_running(0): 0
> rt_nr_running(1): 2
> rt_nr_running(2): 4
> rt_nr_running(3): 0
> rt_overload: 2
> ...
> rt_overload_schedule: 57791
> rt_overload_wakeup:   158650
> rt_overload_pulled:   13722964
> rt_nr_running(0): 0
> rt_nr_running(1): 2
> rt_nr_running(2): 0
> rt_nr_running(3): 3
> rt_overload: 2
> ...
> rt_overload_schedule: 57808
> rt_overload_wakeup:   166924
> rt_overload_pulled:   13722973
> rt_nr_running(0): 0
> rt_nr_running(1): 0
> rt_nr_running(2): 0
> rt_nr_running(3): 2
> rt_overload: 1
> rt_overload_schedule: 57808
> rt_overload_wakeup:   166927
> rt_overload_pulled:   13722973
> rt_nr_running(0): 0
> rt_nr_running(1): 0
> rt_nr_running(2): 0
> rt_nr_running(3): 0
> rt_overload: 0
>
> ---------------------
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