Re: Why is real time pinging not possible with RT kernels?

Chris Friesen
2017-06-12T05:10:51Z

On 06/10/2017 12:13 PM, rolf.freitag@email.de wrote:
> Hi,
>
> I tried ping as a minimal latency test, e. g.
>
> ionice -c3 -p $$
> renice +19 -p $$
> ping -q -s 28 -l 1 -p 0f1e2d3c4b5a6978 -i 0.001 localhost
>
> but when cyclictest shows a worst-case latency of 40 mikroseconds,
> ping shows more than 10,000 (after 1 day run).
> I tried different kernels, e. g.
> SMP PREEMPT RT Debian 4.6.4-1~bpo8+1 (2016-08-11) x86_64
> but with the same result, low cyclictest values, high ping values.
> What is the reason?

That's a great question, I'm curious what responses you get.

It might be useful to use a custom ping client (or remove the -q option and
parse the output) so you could track the latencies and generate a histogram to
see if that 10ms latency was a single outlier or whether it happened fairly
frequently.

Chris