Re: [Xenomai] CPU occupation without context switches?

[Philippe Gerum <rpm@xenomai.org>]

On 05/23/2012 10:31 AM, Arnout Vandecappelle wrote:
> On 05/22/12 21:44, Philippe Gerum wrote:
>> On 05/22/2012 06:09 PM, Arnout Vandecappelle wrote:
>>> Hoi,
>>>
>>> After a few minutes of running my application, I see this:
>>>
>>> # cat /proc/xenomai/stat
>>> CPU PID MSW CSW PF STAT %CPU NAME
>>> 1 828 22 65 0 00300182 9.8 bench_RTnet_scope_thread_loop
>>> 1 839 2 46627538 0 00300186 53.8 bench_RTnet
>>> ...
>>>
>>> i.e. the bench_RTnet_scope_thread_loop takes 10% CPU but no
>>> context switches. How is this possible? I've looked at the
>>> source code and can't find an explanation: when the exectime
>>> accounting is updated, the csw is incremented as well (in
>>> __xnpod_schedule()).
>>>
>>
>> Unless your thread which seems to repeatedly attempt to pend on some
>> sync object has its wait condition satisfied on
>> entry to the syscall most of the time, which may cause
>> __xnpod_schedule() to leave it running without incrementing the
>> switch count.
>
> It is indeed possible that my application does something like that. But
> wouldn't the switch count be incremented every time the bench_RTnet thread
> is executed? That thread runs at 20kHz and at higher priority so it should
> pre-empt the other one, which should lead to context switches in the _loop
> thread.

20Khz does not put much pressure on mid/high end x86, so I believe 
bench_RTnet does not preempt the loop thread that much actually, but the 
latter does consume most of the 500 us time slot without suspending, 
with the former switching in in the remaining quantum most of the time 
(i.e. not preempting any other RT thread).

>
> Unfortunately, I can't reproduce the situation anymore... If I can I'll
> try with Xenomai 2.6.0.
>
>
> Regards,
> Arnout
>


-- 
Philippe.