Re: [Adeos-main] Domain switching latencies

[poornima r <rpoornar@domain.hid>]

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Hello Philippe,
   
  Thanks very much for the reply.
   
  Thanks and Regards,
  Poornima
  

Philippe Gerum <rpm@xenomai.org> wrote:
  On Thu, 2007-06-21 at 05:59 -0700, poornima r wrote:
> 
> We are very thankful to you for providing help and
> guidance in running latency testsuites and other
> assignments 
> We are trying to measure context switching latencies
> between primary and secondary domains with changes in
> the
> latency code as specified in the below link
> http://www.mail-archive.com/xenomai@xenomai.org
> 
> The following changes are done in the latency code and
> the applications are attached
> -------------------------------------------------------
> 1:Measured using linux call printf
> err = rt_task_wait_period(&ov);
> before = rt_timer_tsc();
> printf(" ");

You are measuring time for buffered I/O operations, so your measure is
fundamentally flawed, since this jump won't ever jump to kernel space
unless the buffer gets full once in a while, and the stdio layer has to
sync. At the very least, switch buffering off using setbuf(), give a
newline into the string (depends on underlying termio setup in effect,
so this is unreliable) or simply invoke an actual system call, like
getpgrp() or getppid (and _not_ getpid() which is optimized using direct
fetches from PTDs).

> after = rt_timer_tsc();
> dt = (long)(after - before);
> --------------------------------------------------------
> 2. Measure using Native API call
> err = rt_task_wait_period(&ov);
> before = rt_timer_tsc();
> rt_task_set_mode (T_PRIMARY, NULL, NULL);
> after = rt_timer_tsc();
> dt = (long)(after - before);
> ---------------------------------------------------------
> The worst case latencies was around 20 us using printf
> call and around 60 us when rt_task_set_mode was called
> 
-- 
Philippe.




       
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