Re: 2.6.20.3-rt8 - DMA suffers excessive delay periodically

[Lee Revell]

On 3/27/07, John <linux.kernel@xxxxxxx> wrote:
> I'm runnning 2.6.20.3 patched with -rt8 (and glibc 2.3.6).
> http://people.redhat.com/mingo/realtime-preempt/older/patch-2.6.20-rt8
>
> 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
inoperable.

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

Lee
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/