Latency on Intel Atom D2550 based industrial PC

Hi Guys,

I have tried everything under the sun but my latency keeps well above 70k. The processor has 2 cores (4 if hyperthreading is enabled). I have enabled hyperthreading since the system wasn't numbering the CPUs correctly (htop was reporting CPU 1,3 instead of 1,2). With hyperthreading at least the numbering is correct (1,2,3,4).

I used various isolcpus setups but I have the same issue. For example, isolating CPUs 3 (isolcpus=3) I expect this to be the CPU where the RTAPI processes will run (I read somewhere that RT processes run on the highest CPU number). When I run latency-test I get 4 RTAPI processes but they are scattered across CPUs 3 and 4. Why is this?

Please see screenshots of htop before and after running latency-test (only RTAPI processes are shown for clarity).


My configuration is as follows:
Release Linux Mint 18.1 Serena 32-bit
Kernel Linux 4.9.4-rt2 i686
Intel® Atom™ CPU D2550 @ 1.86GHz × 4
LinuxCNC 2.8 pre
Linux lcnc 4.9.4-rt2 #1 SMP PREEMPT RT Tue Oct 2 01:31:44 EEST 2018 i686 i686 i686 GNU/Linux

Thanks,
Dimos

I'm pretty sure the latency will be hopeless so long as hyper-threading is enabled. (It's akin to trying to run realtime on a virtual machine.)

Different hardware but I found latency increased dramatically (to be unusable) on 32 bit system when running PREEMPT RT kernel.

Sure, I only re-enabled MT after hopeless attempts without it. However, even when running without MT (2 CPUs) and using solatecpus=1 the four TRAPI processes would run on both processors. Even when I used CPU affinity (taskset 0x00000002 latency-test).

I am not sure that isolcpus helps with the prrempt-rt kernel.

Have you tried booting off of the RTAI live-CD and doing a latency test? You can do this without destroying your current install.

I would agree that RTAI is a better choice than Preempt-RT for a slow CPU like the D2550,
especially if you expect to do software step generation.

That said, your latency test shows > 1 ms latency.

Latency this bad is often related to power management so I would make sure all power
management options are disabled in the BIOS (no turbo mode, no CStates >1 etc etc)