Hi Philippe,
>
> I'm not sure the cost is as much as 2 us, unless you don't use the
> sysenter/sysexit protocol for syscalls Xenomai manages properly
> (--enable-x86-sep). It seems you are going through the ancient 0x80
> exception vector, but still, I agree that the point remains: no
> contention should mean no transition to kernel.
>
I did some measurements to get precise values here.
For this I used the following Xenomai program that accesses a semaphore twice
within a loop (p followed by v).
---------------------------------
#include <native/task.h>
#include <native/sem.h>
RT_TASK taska_desc;
RT_SEM sem;
void mytaska(void *cookie)
{
int i;
RTIME tim1, tim2;
#define LOOPS 100000
tim1 = rt_timer_read();
for (i=0; i < LOOPS; i++)
{
rt_sem_p(&sem, TM_INFINITE);
rt_sem_v(&sem);
}
tim2 = rt_timer_read();
printf("delta is %llu per step: %llu\n",
tim2-tim1,
(tim2-tim1)/ LOOPS);
}
int main(void)
{
mlockall(MCL_CURRENT|MCL_FUTURE);
rt_sem_create(&sem, "mysem", 1 ,0);
rt_task_create(&taska_desc, "mytaska", 0, 81, T_JOINABLE);
rt_task_start(&taska_desc, &mytaska, NULL);
rt_task_join(&taska_desc);
rt_sem_delete(&sem);
return 0;
}
-----------------------------
I measured the following runtime:
For configuring Xenomai without any option: 4.8us per step
Configuring Xenomai with --enable-x86-sep: 4.5us per step.
I ran this experiment on a 3.2 GHz Pentium D on a server main board with
E7230 chipset.
Xenomai 2.3.1, kernel 2.6.20.4, SMP
Thus the performance here is not really excellent (as there is no need to do
a task switch.
I agree with all the comments that recommended to improve the design first
before
trying to improve the OS performance.
However, if there is (RT)OS, I expect it to be as fast as possible.
Because, even if I have a "perfect" design, I need the OS - otherwise I would
just
live without the OS.
A better design could help to reduce OS calls but it can not avoid it
completely.
Also, a lock less designs might lead to additional data copies which is also a
known
performance killer. So I think this is a decision that has to be made on base
of the concrete
project. General rules do not help here.
Thanks for all comments!
Regards
Mathias
--
Mathias Koehrer
[EMAIL PROTECTED]
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