Dual-Ported Memory & Shared Memory Support

Joel Sherrill joel.sherrill at OARcorp.com
Fri Oct 18 18:17:44 UTC 2002

Salman wrote:
> Hey guys,
> I'm back from my summer holidays, and now have to seriously dig into this
> RTEMS over SCI cluster project.
> I need to do address translation between SCI network and internal addresses,
> modification of Dual-Ported Memory Manager should enable me to do this.

You shouldn't have to modify the DP Memory driver.  You should only
have to create a DP memory object to do the conversion.  What do you
think is missing?

> But what I don't understand is the Shared Memory Support. Can someone tell
> me what was this Shared Memory Support designed for ? and whether I would
> need it or not for SCI network.

The Shared Memory MPCI driver assumes that the CPUs have some shared 
memory to place inter-processor traffic in.  If SCI does not satisfy
requirement, then it is not for this situation.

> Aside from this, can I have your opinions on Message passing over Ethernet.
> If porting of SCI drivers is to be very time-consuming (which by the looks
> of the driver source code, it is), we might consider having RTEMS on an
> Ethernet based computer cluster (much slower obviously).

If you end up with a node-interconnect that is message passing, then
I would write a new MPCI driver which had TX and RX server tasks.
The RX server would block for requests to arrive and then invoke
mpci_announce.  The TX server would do all TX's to other nodes.
WHen RTEMS does a MPCI send packet, it should return ASAP!!! and
basically queue up a request for the TX server to do the real work.
When using the shared memory driver, TX requests happen quickly
enough to avoid this middle layer.

Does that make sense?

> Thanks,
> Salman
> ------------------------
> Fianl Year
> Computer & Electronic Engineering student
> Trinity College, Dublin
> ------------------------

Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
Support Available                (256) 722-9985

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