Hello!
I need use sleep with accurat timing.
I use 2.6.21 with rt-prempt patch.
with enabled rt_preempt, dyn_ticks, and local_apic
But
req.tv_nsec = 300000;
req.tv_sec = 0;
nanosleep(&req,NULL)
make pause around 310-330 microseconds.
I tried to understend how work nanosleep(), but it not depends from
jiffies and from smp_apic_timer_interrupt.
When can accuracy be lost?
And how are process waked up?
GolovaSteek
GolovaSteek skrev:
> Hello!
> I need use sleep with accurat timing.
> I use 2.6.21 with rt-prempt patch.
> with enabled rt_preempt, dyn_ticks, and local_apic
> But
>
> req.tv_nsec = 300000;
> req.tv_sec = 0;
> nanosleep(&req,NULL)
>
> make pause around 310-330 microseconds.
How do you measure this?
If you want to have something done every 300 microseconds, you must not
sleep for 300 microseconds in each iteration, because you'd accumulate
errors. Use a periodic timer or use the current time to compute how long
to sleep in each iteration. Take a look how cyclictest does it.
> I tried to understend how work nanosleep(), but it not depends from
> jiffies and from smp_apic_timer_interrupt.
>
> When can accuracy be lost?
> And how are process waked up?
>
>
> GolovaSteek
Don't forget the process will always have non-zero wakeup latency. It
takes some time to process an interrupt, wakeup the process and schedule
it to run on the CPU. 10-30 microseconds is not unreasonable.
Michal
2007/8/17, Michal Schmidt <[email protected]>:
> GolovaSteek skrev:
> > Hello!
> > I need use sleep with accurat timing.
> > I use 2.6.21 with rt-prempt patch.
> > with enabled rt_preempt, dyn_ticks, and local_apic
> > But
> >
> > req.tv_nsec = 300000;
> > req.tv_sec = 0;
> > nanosleep(&req,NULL)
> >
> > make pause around 310-330 microseconds.
>
> How do you measure this?
> If you want to have something done every 300 microseconds, you must not
> sleep for 300 microseconds in each iteration, because you'd accumulate
> errors. Use a periodic timer or use the current time to compute how long
> to sleep in each iteration. Take a look how cyclictest does it.
no. I just want my programm go to sleep sometimes and wake up in correct time.
> > I tried to understend how work nanosleep(), but it not depends from
> > jiffies and from smp_apic_timer_interrupt.
> >
> > When can accuracy be lost?
> > And how are process waked up?
> >
> >
> > GolovaSteek
>
> Don't forget the process will always have non-zero wakeup latency. It
> takes some time to process an interrupt, wakeup the process and schedule
> it to run on the CPU. 10-30 microseconds is not unreasonable.
But 20000 operations can be done in 10 microseconds?
and why is there that inconstancy? Why sametimes 10 and sometimes 30?
In which points of implementation it happens?
GolovaSteek
On Aug 17 2007 11:44, GolovaSteek wrote:
>> How do you measure this?
>> If you want to have something done every 300 microseconds, you must not
>> sleep for 300 microseconds in each iteration, because you'd accumulate
>> errors. Use a periodic timer or use the current time to compute how long
>> to sleep in each iteration. Take a look how cyclictest does it.
>
>no. I just want my programm go to sleep sometimes and wake up in correct time.
Would it be acceptable to use an optimistic strategy, like the one below?
Let's say that the following tasks happen at each time: A at 0, B at 300, C at
600, D at 900, E at 1200, F at 1500. Assume sleeping takes 500 µs.
Then B and C could be run at 500, D at 1000 and E,F at 1500.
Jan
--
GolovaSteek wrote:
> 2007/8/17, Michal Schmidt <[email protected]>:
>
>> GolovaSteek skrev:
>>
>>> Hello!
>>> I need use sleep with accurat timing.
>>> I use 2.6.21 with rt-prempt patch.
>>> with enabled rt_preempt, dyn_ticks, and local_apic
>>> But
>>>
>>> req.tv_nsec = 300000;
>>> req.tv_sec = 0;
>>> nanosleep(&req,NULL)
>>>
>>> make pause around 310-330 microseconds.
>>>
>> How do you measure this?
>> If you want to have something done every 300 microseconds, you must not
>> sleep for 300 microseconds in each iteration, because you'd accumulate
>> errors. Use a periodic timer or use the current time to compute how long
>> to sleep in each iteration. Take a look how cyclictest does it.
>>
>
> no. I just want my programm go to sleep sometimes and wake up in correct time.
>
What does your program do that it has such a strict requirement on the
exact length of sleeping?
>>> I tried to understend how work nanosleep(), but it not depends from
>>> jiffies and from smp_apic_timer_interrupt.
>>>
>>> When can accuracy be lost?
>>> And how are process waked up?
>>>
>>>
>>> GolovaSteek
>>>
>> Don't forget the process will always have non-zero wakeup latency. It
>> takes some time to process an interrupt, wakeup the process and schedule
>> it to run on the CPU. 10-30 microseconds is not unreasonable.
>>
>
> But 20000 operations can be done in 10 microseconds?
> and why is there that inconstancy? Why sametimes 10 and sometimes 30?
> In which points of implementation it happens?
>
> GolovaSteek
>
If a jitter of 20 microseconds is unacceptable for your application,
don't use PC hardware. Consider using a microcontroller.
Michal