Poor Latency and Fix for New AMD APU's

There a current problem with latency on newer AMD APU's that use the newer Northern Islands and newer graphics cores.

See this wiki for a table of cores, chipset and product names.
wiki.gentoo.org/wiki/Radeon

Kernel Mode Settings (KMS) needs to be turned off or you will experience large latency spikes.

The kernel command line parameter is: radeon.modeset=0

This disables the hardware graphics drivers and the llvmpipe driver is used instead. Typical max latency might be 200-300 uS with KMS on and ~20 uS with it off using an RTAI kernel. The latency behaviour is similar with preempt_rt and KMS. The problem does not appear if the older 2.6.32 RTAI kernel is used, since KMS for the newer chipsets weren't in that older kernel.

We are trying to find out what the problem is and fix the hardware drivers. If you experience different behaviour please let us know.

Another fix for poor latency is adding the isolcpus= kernel parameter with RTAI kernels. RTAI has a broken scheduler for the time being so this looks a little backwards, but the working setting:for a 4 core APU is: isolcpus=0,1,2

This should be used along with turning off kernel mode settings: radeon.modeset=0

I'll test the 2 core APU's soon and post an update if isolcpus=0 or if isolcpus=1 act the same or if one has more benefit than the other.

We did some further testing with different isolcpus= settings and the results are on the wiki
wiki.linuxcnc.org/cgi-bin/wiki.pl?The_Is...RUB2#Latency_Testing

The advice is to isolate both CPUs in the pair for a 4 core APU. Isolcpus=2,3

We also confirmed that the latency-test and the rtai kern/latency test select cpu' cores differently.

You want it to use cpu 0 if there is only cpu 0; cpu 1 if there are cpu 0 and cpu 1; and cpu 2 if there are cpu 0,1,2 or more cores.

The recommended kernel parameters are: radeon.modeset=o isolcpus=2,3

There is a new scheduler in 3.14 RTAI. It no longer requires any isolcpus kernel parameter to be set.

See this thread for more info:
www.linuxcnc.org/index.php/english/forum...314-rtai-now-working