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

[Mike Marty <mikem@xxxxxxxxxxx>]

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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