Re: - DMA suffers excessive delay periodically

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On 3/27/07, John <linux.kernel@xxxxxxx> wrote:
I'm runnning patched with -rt8 (and glibc 2.3.6).

I've written a program to highlight a phenomenon I don't understand.

This system includes a PCI board that provides data at ~38 Mbit/s.
I request data in lumps of 1316 bytes.
In other words, a new request should complete every 277 µs.

However, if I time stamp each packet as soon as I receive it, I notice
that almost exactly every 2 seconds, the packet is received 30-100
microseconds late.

Sounds like periodic SMM traps.  There's not much you can do about
them other than to avoid such hardware if you have tight RT
constraints.  The RTAI people have some code to disable SMM but it
does not work on all chipsets and may render some of your hardware

Is this a laptop?  They are plagued with SMM problems...

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