How to track down latency issues?

Hi,

I recently bought a IBM M51 small factor computer, and it appears to have latency issues.

In fact, the average latency is not bad (15-20µs), but I get some pikes at 300µs.

These pikes occures every 32-33s. So far, I didn't find what is the problem. I tried to switch off components from BIOS, tried to use a PCI video card (there is no option in the BIOS to disable the internal one, but it seems to automatically switch to the external one), use vesa driver... No change.

So, I'm wondering what can I do to track down that problem? Are there some tools to record all interrupts, and see the one leading to a freeze for unusual delay?

BTW, does such latency issue matter if using an external step/dir/pwm generator, like a MESA board? Does it matter which one I use (PCI/parport)?

Thanks for you help.

Have you looked here...
wiki.linuxcnc.org/emcinfo.pl?TroubleShooting
wiki.linuxcnc.org/emcinfo.pl?FixingSMIIssues

Rick G

Ok, I found something interesting.

I played arround with the SMI bits, to mask some SMI features. And it appears that if I mask the PERIODIC_EN bit, no latency pikes occur anymore.

Digging in the ICH6 datasheet (786 pages which are total chineese for me), I found that this bit enables a periodic interrupt, and the rate of this interrupt can be 1min, 32s, 16s or 8s!!! Obviously, my config is set to interrupts every 32s...

The question, now, is: what happens if I let this bit masked? What is this periodic interrupt for?

you may find something here: en.wikipedia.org/wiki/System_Management_Mode

Thanks for the links.

It seems that the SMI# interrupt is use by the BIOS to do a lot of things. But the BIOS internals are not documented, so it is very hard to know what they do.

I will try to disable that periodic interrupt, and see what are the consequences. My main fear is about thermal management of the processor... Hard to know if the CPU will switch down to avoid burning before it actually burns! I will juste be able to check if the fans are still correctly controlled while CPU usage increases.

I have also switched off SMI at an old laptop with latency around 1 msec (before switch off).
I had the same concern as you, but never any serious problem.
And I never saw a report about real issues, although the SMI issue is common as latency killer, and I guess frequently switched off.

Gerd

Ok, that's good news!