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@xxxxxxxx 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
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