Re: Measuring timer interrupt latency

Tim Sander
2009-04-13T16:22:09Z

Hi
> Have you tried cyclictest? It's part of the rt-tests package that
> Thomas Gleixner wrote and that I now maintain.
I think cyclictest is nice to get some first impressions and i really like
it... but it only give the aggregate.

> The thing is, I'm not sure how useful measuring the interval between
> interrupt and ISR. It's such a small component of the total latency
> value that it usually gets lost in the noise.
I don't think that you can neglect these numbers in a hard rt environment.
As far as i understand, the irq-tasklet has to be scheduled by the kernel.
> If you are wanting to decompose the latency into it's components on RT,
> I think you'd have:
> 1. ISR latency
> 2. Scheduler latency for IRQ thread
> 3. IRQ handler time
> 4. Scheduler latency for user-space thread
> Cyclictest measures the aggregate of all these.
One good aproach to minimize user mode latency are probably user mode drivers.
This approach has been presented at the Embedded in Nürnberg this year by
linutronix. So you can shave off #2 of the above list. Which means only one
rescheduling runs instead of two. Usermode has of course to be run with rt
priority.

Tim
--
''=~('(?{'.('._@@[~'^'^-)./^').'"'.('}@@~-]@_^__'^'))-^~<.;;-}').',$/})')
--
To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html