Latency Results for Real Time Servo Conf.

Ok I just got my Atom board up and running and ran the latency tester while running glxgears, browsing this forum, installing openofice. Here are my results:

Servo Thread; Max int (ns) 1010484 max jitter: 12084
Base Thread: Max int (ns) 37964 Max Jitter: 14160

I ran this for 15 minutes and going to leave it running overnight. I'm wanting to know if I'm going to be ok with these numbers running a closed loop realtime servo config? I don't have my servo hardware yet but it's in the mail.

I think you need to turn Hyper threading off and set isolcpus=1 to get the lower latency
but the trade-off with isolcpus=1 is that user programs only have 1 CPU so will run slower.
12 usec latency is fine for a servo system so a little more latency with snappier user interface
is probably a good tradeoff

BigJohnT wrote:

If it is a D510MO the latency should be lower... but that is fine for a servo machine using something like a Mesa 5i20 + 7i33 +7i37 type of setup.

John

It is the D510MO. I just upped the ram timing to 5-5-5-15 and it didn't effect anything. This is also with HT turned off. I'm not to sure if an old SATA HDD would effect anything. I also have a PP Card installed in the PCI slot. I had it their just so the drivers would be installed so if I had to use it on down the road. Tomarrow I'm going to get a 16GB USB 2.0 thumb drive and see how it runs with no HDD. The HDD is so damn slow. It's a 250GB Deskstar. I'm not to sure how to implament the cpu change. It is a duel core and I don't mind dedicating one core just for EMC2 cause this machine is my CNC Machine.

The end of this thread has the instructions for isolcpus:
www.linuxcnc.org/component/option,com_ku...0/lang,english/#6955

So is it the max Jitter that we are concerned with? I think this Atom board has better jitter than my 975 P4C board. TY for the info for the ISOcpu. I'm going to get on it very soon to see if I can't get some better numbers.

Ok by isolating one CPU it is far better, but I can only open 2 instances of glxgears and nothing else. I have to close one in order to open anything else. I saw mention of using control groups and containers instead of isolation but It seems pretty complicated. Anyway here are my new latency scores.

Servo Thread 1,003,677 Max Jitter 5,432
Base Thread 31,215 Max Jitter 6,255

It's much better.

Its much better latency wise but worse in terms of responsiveness
So for a software generated step machine it might be a good compromise
since that low a latency will allow you to generate quite high step rates via a
parallel port.

On the other hand, for a servo machine even 20 uSec or so of latency
would hardly be noticeable so you might want the better userland responsiveness
instead of minimum jitter in that case

ty for the comment. I think I'll try it out with both CPU's running and see how ti works then try the ISO once I have the servo's tuned to see if their is any difference. I have a feeling their isn't going to be much of a noticable difference either.

On a side note. I just removed my HDD and am using a USB 2.0 8GB stick. I'm not to sure if it's any faster It feels like it but it actually made my latency a little worse. New scores with fresh install. This is running 3x glxgears, update manager, Several FF windows.

Servo: 1,014,461 Max Jitter: 16459
Base: 42325 Max Jitter: 18576

It's a little strange but I'm also trying to minimize heat and not using a HDD is definitely going to cut down on heat production. Well actually 3x glxgears is kepting both CPU's at 90%+ Load. And the system seems to bring up icons and menus faster than with the old HDD. I think with a little tweaking of Ubuntu I can get his pretty fast and low latency. I'm going to look into using control containers for EMC2. Maybe I can get the best of both worlds.

A few questions. How do I disable disk swap? I don't want to use it with a Memory stick. It's write is just way to slow for that.

NM. I found it. Very easy to do with Gparted. Just right click on the swap partition and disable. Easy peazy.

Well with isolation and the USB flash drive running 2 instances of glxgears for 2 days along with Firefox on a active page that changes a lot. Here are my results.

Servo: 1002261 Jitter: 7811
Base: 33045 Jitter: 8721

The system really isn't to bad to use in this state. I'm going to start up with this configuration. Just wanted to kept a tab on my results. I'm surprised on the jitter but this thing has been running 100% Load for over 2 days and it took almost 10 minutes for the screen lock password to even come up it was so overloaded. I'm happy as sin personally. I also think I can saftly use these numbers without fear of any surprises.