Re: Seeing 800us of latency with cyclictest on a PC with 2.6.26.8 rt patch. Is that normal?

[Clark Williams <williams@redhat.com>]

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On Wed, 11 Mar 2009 19:31:01 +0000 (UTC)
Brian Hutchinson <b.hutchman@gmail.com> wrote:
> 
> Thanks to everyone for all the suggestions.  I wouldn't have thought that 10
> year old desktop hardware could beat the pants off a modern laptop.
> 
>

Almost certainly it's because your old desktop doesn't have lots of
things going on behind-the-scenes in the BIOS. Modern x86/x86_64 h/w
vendors have added lots of outside the OS functions such as thermal
monitoring and special function keys (ala the Thinklight on most
Thinkpads) that are handled via System Management Interrupts (SMI)
directly by the BIOS. If the SMI service routines take long times to do
whatever they're doing, you get unexplained latencies. 

Jon Masters wrote an SMI detector module that we've been using to good
effect to detect and classify SMIs (/me rummages for LKML link).
Actually, I think we've added some stuff since he posted it last, so
I'll poke him to post it again. It's a special purpose module that runs
a function in the stop_machine() mode. The function just polls the TSC
looking for gaps that exceed a certain threshold. Any gap in the TSC
values on a particular core are very good candidates for SMIs.

Clark
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