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

Corey Minyard
2017-06-12T12:57:21Z

On 06/10/2017 01:13 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?

Since no one else has answered, I guess I will.

Creating a real-time *system* is hard work. Just running a program on a
real-time OS does not magically make it a real-time system. You really
have to know what you are doing. I suggest you read "POSIX.4
Programmers Guide: Programming for the Real World" for an intro to
the subject. It's fairly close to what Linux does, though there are things
that Linux does beyond what is in there.

A few notes on what you have done:

* ionice doesn't affect network I/O.
* renice does not set real-time priorities.
* You don't do anything with memory locking. What happens
if memory in your program gets paged out then is required?
* Something on the other end of the ping is running, too. It
also needs to be real-time, too, and it's not by default.

I'd suggest you need to spend some time learning before you attempt
something like this.

-corey