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
_______________________________________________
Beowulf mailing list, Beowulf@beowulf.org
To change your subscription (digest mode or unsubscribe) visit
http://www.beowulf.org/mailman/listinfo/beowulf
_______________________________________________
Beowulf mailing list, Beowulf@beowulf.org
To change your subscription (digest mode or unsubscribe) visit
http://www.beowulf.org/mailman/listinfo/beowulf
