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

Chris Friesen
2017-06-12T16:45:40Z

On 06/12/2017 06:57 AM, Corey Minyard wrote:
> 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.

Good point...the OP probably wants something like "sudo chrt <prio> <cmd> ...."

> * You don't do anything with memory locking. What happens
> if memory in your program gets paged out then is required?

I suppose this would require a custom "ping" client to ensure all necessary
memory is allocated and locked before starting the actual ping.

> * 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.

The standard ICMP ping would be handled by the kernel. With linux-rt I guess
you'd have the ethernet interrupt thread involved as well as ksoftirqd?

Chris