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

Corey Minyard
2017-06-12T17:16:47Z

On 06/12/2017 11:45 AM, Chris Friesen wrote:
> 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.
>

Yes, I don't know if there is a way to do this without doing it in the
application.

>> * 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?
>

I didn't look at what would be required, but those are likely candidates.

I didn't mean the above as a comprehensive list or anything like that.
I just
wanted to drive home the point that this requires end-to-end thinking
and careful design. It's hard.

-corey