Re: [Gems-users] Message size and network latency

[Mike Marty <mikem@xxxxxxxxxxx>]

Also you might just consider changing Throttle.C to divide the size of the
message by the width of the link.  Add this result to the fixed enqueue
latency.  This approximation might serve your needs.

--Mike

> This is a limitation of the simple networking code.  The MessageSize
> certainly affects the bandwidth of a link and therefore can cause
> subsequent messages to incur queuing delay.  However if the size of a
> message exceeds the size of a link, then it doesn't incur additional
> queuing delay for _that_ message.
>
> It is definitely an area of GEMS that needs improvement I think.  On the
> other hand, usually the only messages that exceed the link size are data
> messages, and it is reasonable to assume that the critical word arrives
> first?
>
> Thus the latency of a message is affected by queuing delays and the fixed
> latencies of when a message is enqueued to the next link on the
> interconnect.  These fixed latencies are specified in the Slicc file when
> injecting the message into the network and the fixed latencies specified
> for each link in the interconnect.  The latter are either specified for
> each link using a FILE_SPECIFIED network, or the NETWORK_LINK_LATENCY and
> ONCHIP_LINK_LATENCY parameters for the auto-generated topologies.
>
> You might want to e-mail Valentin Puente at the University of Cantabria.
> At one time he integrated his more-detailed interconnect simulator into
> Ruby.  Unfortunately for me, I cannot read Spanish code and comments, but
> this shouldn't be a problem for you ;-)
>
> --Mike
>
>
> > Dear list,
> >
> > I have noticed that changing the message size of some messages does
> > not affect the network latency for that message.
> >
> > For example in the MOESI_CMP_directory coherence protocol, I have
> > changed the message size from "MessageSizeType:ResponseLocal_Data" to
> > "MessageSizeType:Response_Control", and I have measured the latency of
> > the message before and after the change, finding the same result (8
> > cycles).
> >
> > How does the MessageSize field affect the behaviour of the network?
> > What other parameters affects network latency?
> >
> > Thanks in advance,
> >
> > Alberto Ros.
> >
> >
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