Re: preempt-rt, NUMA and strange latency traces

[Sébastien Dugué <sebastien.dugue@bull.net>]

On Thu, 2006-02-09 at 10:54 -0800, john stultz wrote:
> >
> >   I think it's an RSA1 card, but no certainty here. I don't think
> > the x440 came with the RSA2. Is there a way to check for sure without
> > unplugging everything (It's such a mess of cables behind the box)?
> 
> No, you're right, the x445 was the box that could have either RSA1 or
> RSA2. The x440 can only have an RSA1.
> 
> 
> >   Whats the SMI issue with the RSA? Could the RSA generate bursts of SMI
> > that could be enough to freeze the CPUs?
> 
> SMIs effectively freeze all cpus while the BIOS code executes. With the
> RSA1, there are two main causes of SMIs:
> 1) Periodic SMI: Which occurs every 15 minutes as the RSA checks various
> hardware status and lasts for ~30ms.
> 
> 2) Console Redirection SMIs: This occurs only when you use the console
> redirection feature from the RSA. You'll see ~30ms stalls ~once a second
> while the SMI screen-scrapes the console text buffer and sends it out
> over the serial line.

  That must be what I've been hitting in my tests and will check soon.

> 
> The console redirection one is easy to avoid: just don't use that
> feature if you care about low latencies. The periodic SMI is more
> difficult to work around. With RSA2 based systems, there is a BIOS
> update that allows you to disable this functionality (and on newer
> systems it ships disabled), however I don't believe there is any such
> feature for the older RSA1 based systems.

  I'll have to live with it then.

> 
> 
> >   When I ran those tests I was logged on the RSA (serial line). I will
> > try to run the tests again without being connected when I can manage to
> > get some CPU time (eh! shared machine)...
> 
> Yes, not using the console redirection feature will definitly help the
> situation, but there still will be the possibility of ~30ms stalls every
> 15 minutes on the x440.

  That's the problem and I'll have to synchronize for a proper window
for running my tests.

  Is this SMI thing IBM's eyes only stuff or is it documented somewhere?


  Thanks for that valuable info.

  Sébastien.



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