Albert D. Cahalan writes:
>
> Richard Gooch writes:
> > mauro@alusis.ucb.edu.bo writes:
> >> Hi friends!
> >>
> >> I'm trying to develop a Dristributed Shared Memory sytem(DSM) using
> >> Linux. I was wondering myself if using IP for the protocol of the
> >> comunicationg nodes were correct?. I thing using IP will add so much
> >> bytes to the messages(actually packets) sent by the nodes. Will be
> >> better try another lighter protocol?. Could you also point me to
> >> other similar projects that are being develop? it will clear my
> >> ideas and help me with my problems
> >
> > Ach! Not another DSM project :-( Don't do it. There's already a DSM
> > implementation for Linux,
>
> Obviously this is a feature people want.
Too many people think they want it because they don't really
understand the alternatives and the implications.
> > and besides, you're better off with a message-passing interface.
> > That way application coders can see how costly operations are.
> > Using DSM hides that, resulting in inefficient code.
>
> Message passing can be more costly! On the hardware I develop for, a
> "message" involves setting up some DMA control data. Distributed
> shared memory has a one-time setup cost, so it is faster for
> frequent access to small bits of data.
Message passing must be more efficient than DSM. We are talking about
clusters, not a single SMP machine. The reason is that DSM *must* sit
on top of a transport layer (aka message passing).
> You could really mess up performance by using a message-passing API
> for repeated random access to 8-byte values. Actually, I think the
> break-even point is near 2 kB.
If you mean within a single, (hardware) shared memory computer, then
yes. Otherwise, no. And DSM is all about pretending you have shared
memory across a network of computers.
Regards,
Richard....
Permanent: rgooch@atnf.csiro.au
Current: rgooch@ras.ucalgary.ca
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