Re: nanosleep() accuracy

From: Michal Schmidt
Date: Fri Aug 17 2007 - 03:23:08 EST


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
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