[Xenomai-help] First loop iteration increased latency

[Daniele Nicolodi <daniele@domain.hid>]

Hello. I'm obtaining quite nice results with Xernomai in my data
acquisition program. However I observe an unexpected behaviour.

My test application works as follow: one process puts a data frame into
a shared memory region and increments a semaphore. Another process waits
on the semaphore, takes the data frame from the shared memory and
processes it. I also place in the shared memory the time (obtained from
clock_gettime(CLOCK_REALTIME, &ts)) the time at which I increment the
semaphore. In the consumer process I look at the time difference between
the stored time and the time at which I read the frame from the shared
memory. In this way I hope to obtain a measurement of the inter process
communication latency.

I obtain a quite satisfactory latency, in the 20-30 usec range. However
the latency for receving the first frame is always much larger: in the
order of 8-10 msec! What can be the cause of such latency?

I'm monitoring xenomai mode switches with pthread_set_mode_np(0,
PTHREAD_WARNSW) and I observe none of those. I thought that maybe I'm
encountering page faults when reading for the first time the shared
memory segment and I tried to "initialize" the memory reading from it in
a very stupid loop:

int i;
char c;
from (i = 0; i < SIZE; i++)
	c = ((char *)shm)[i];

but it can be that the compiler is smarter than me and is optimizing the
loop away. What is a better way to issue the page faults in a non
critical section of my code? It would be nice if xenomai would support
MAP_POPULATE mmap() flag.

How can I track the source of this increased latency?

Thanks. Cheers,
-- 
Daniele