Preempt-RT kernel aceptable latency

Hi,

I installed linux Mint with the Preempt-RT kernel , but I got this latency results:




a bigger latency is expected, but this looks to like can be a problem... My question is, for this kernel,until what latency value its acceptable?

I'm running a mesa 7i76e.


João

I would first try the latency test with the base thread disabled,
since this is the environment (servo thread only) that you will be running with hm2_eth:

latency-test 1000000 1000000

or:

latency-histogram --nobase

400000 (.4 ms) might be barely OK but you wont know till you try
if you cannot improve this with BIOS settings or a more recent kernel
(current Preempt-RT is 4.4.1-rt6) you could probably use a 2 ms servo thread

I'm using the 3.2.0-0.bpo.4-rt-686-pae on a asrock q1900m. can I install thie new kernel on linux mint?

that is the results without base thread:








João

My ASROCK Q1900m-PRO3 does much better than that with Preempt-RT about 15us (and with RTAI it's even better). Are you sure you have all the power saving "features" disabled in the BIOS?

I also have a Q1900 mb and "turning off" 3 cores in linux made a big difference for me. I don't remember how I did, but it should not be too hard to find.
--Per

You can install a new Preempt-RT kernel fairly easily in most systems, but it looks
like your servo thread only latency is fine for a 7I76E at 1 KHz servo thread rate

Note that if you are running a non-parallel port system, latencies are much less important.
you definitely dont want to disable any cores if you are running external stepgens/encoder counters,
as that will just make the system slower, and on real time uspace Ethernet systems will
typically increase the network latency

I was thinking of this page wiki.linuxcnc.org/cgi-bin/wiki.pl?The_Is..._Parameter_And_GRUB2 . when I wrote "turn off core".

I might have to remove this setting then since I will add a stepgen for my toolchanger.


--Per

I have a Asrock Q1900M motherboard and the 1ms servo thread is less than 3570ns with the 25ms base thread at 13624 running two instances of GLXGears.

I have turned all the C state stuff, onboard audio off in the bios. and I am using a ssd hard disk

I installed linuxmint LMDE2 mate 64 bit edition and followed TJ's install here forum.linuxcnc.org/forum/9-installing-li...debian-8-mint-lmde-2

The kernel I am using is 3.18.13-rt10mah with kernel parameters of isolcpus=2,3 intel_idle.max_cstate=0 processor.max_cstate=0

With bios tuning I manage to decrease the latency.





I followed a old (and tested by me) tutorial of TJ's, I should try this new one istead.

sprintertrd, are you using on-board graphics? the machine boots properly with that kernel?


João

Yes I am using the on-board graphics card and allocated 512K memory for it.