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#1 2020-05-06 08:16:48

liborw
Member
Registered: 2020-05-06
Posts: 2

Problem with ping latency

Hi,

I have wired network with two Arch computers (A,B) and a robot (R), the robot control system require very low round trip times for the control packets (< 1 ms). The problem is that  when I ping from A to R the average rtt is around 0.95 ms which doesn't leave much time for computation, however ping from B to R has rtt around 0.35 which should be fine, ping B to A has rtt also around 1.0 ms. The odd thing is that B is old Macbook Pro and A is new freshly installed PC with realtime kernel meant to be used to control the robot. Another odd thing is that we have similar setup with almost the same computer (A) which works fine with rtt around < 0.5 ms.

Any idea where might be a problem, what to check?
Thansk

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#2 2020-05-06 12:15:06

Zod
Member
From: Hoosiertucky
Registered: 2019-03-10
Posts: 343

Re: Problem with ping latency

https://wiki.archlinux.org/index.php/Im … ce#Network

Which, of course, is nonsense. You should be looking at the performance of the actual control packets and/or the protocol it uses.  ICMP probably isn't it.

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#3 2020-05-06 13:49:59

liborw
Member
Registered: 2020-05-06
Posts: 2

Re: Problem with ping latency

Thanks, I have already went through all of these suggestions, but most of them does not impact ICMP protocol. And you are right that ping doesn't test the actual communication, but it is a first and simplest test (also suggested by the robot manufacturer), which failed, therefore I'm starting from that. There are other which tests the actual communication and they also failed (on the latency), but they are not in the scope of this forum.

By the fact that rtt of B is half of that of A I have guessed that it is not problem in the actual network, both are connected to the same switch, got the IP from same DHCP, both has same routing table and only difference I can think of is that B has way longer eth cable.

I already have some luck with ethtool:

sudo ping $ROBOTIP -i 0.001 -D -c 10000 -s 1200
...
rtt min/avg/max/mdev = 0.463/1.524/1.937/0.083 ms

ethtool -C enp1s0 rx-usecs 10 tx-usecs 10 # was 50 and 200

sudo ping $ROBOTIP -i 0.001 -D -c 10000 -s 1200
...
rtt min/avg/max/mdev = 0.588/0.997/1.303/0.047 ms

But that is not enough.

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