[Beowulf] interconnect and compiler ?

[Vincent Diepeveen]

Now that you're busy with all this, mind quoting interconnects switch  
latency?

For example: If one of the cores c0 on our box is busy receiving a  
long message
from a remote node in the network, a message that will take  
significant time,
can it switch in between let go through a short message meant for c1,  
and if so
what latency time does it take to receive it for c1?

The switching between different cores is more important nowadays than  
the actual
latency for a blocked read, as switching seems to be a very weak spot  
in most interconnects.

Thanks,
Vincent

On Jan 30, 2009, at 6:06 PM, Greg Lindahl wrote:

> On Fri, Jan 30, 2009 at 07:07:34AM -0700, Michael H. Frese wrote:
>
>> Johnn Adams said "Facts are stubborn things," and there just aren't
>> enough of them in your example to determine whether bandwidth or  
>> latency
>> dominates communication time.
>
> Mark asked for an example, not a research paper. And we were
> discussing something other than latency and bandwidth, because these 2
> parameters aren't the only fundamental ones for a communications
> network.  In the Berkeley "logp" model, for example, processor
> overhead and the "gap" betweeen messsages are fundamental parameters.
> The InfiniPath chip has a tiny "o" and a negative "g".  As a result,
> it can send a lot more small messages than other interconnects, and
> this number rises as you add more cores to a system (!). Mark was
> wondering when that was important.
>
> Even logp doesn't describe an interconnect that well. It matters how
> efficient your interconnect is at dealing with multiple cores, and the
> number of nodes. As an example of that, MPI implementations for
> InfiniBand generally switch over to higher latency/higher overhead
> mechanisms as the number of nodes in a cluster rises, because the
> lowest latency mechanism at 2 nodes doesn't scale well.
>
> -- greg
>
>
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