Re: preempt-rt, NUMA and strange latency traces

[john stultz <johnstul@us.ibm.com>]

On Thu, 2006-02-09 at 12:11 +0100, Sébastien Dugué wrote:
> On Wed, 2006-02-08 at 08:49 -0800, john stultz wrote:
> > On Wed, 2006-02-08 at 11:45 +0100, Sébastien Dugué wrote:
> > >   The more I think about it, the more I tend to believe it's hardware 
> > > related. It seems as if the CPU just hangs for ~27 ms before
> > > resuming processing.
> > 
> > That sounds like to the ~30ms RSA caused SMIs. Does this system have an
> > RSA1 or RSA2 card?
> > 
>
>   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.

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.


>   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.

thanks
-john

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