[Beowulf] dual-core benefits?
hahn at physics.mcmaster.ca
Fri Sep 23 09:43:17 EDT 2005
> > is this scalability assuming a slow interconnect like gigabit?
> Yes, gigabit on Pentium 4 cluster.
well, the e1000 is a decent nic, but it is sometimes configured
for too much interrupt mitigation to suit HPC. even so, it's not
anywhere close to the domain of performance as a real cluster
interconnect (~2 us, 400-1800 MB/s).
> > have you considered when it would be appropriate to go to something fast?
> Yes, that is probably sth that we will consider after trying gigabit and two
> network interfaces per mb.
dual-port has a reputation for not helping much. it's only a small
boost in bandwidth in the ideal case.
> > on a multiprocessor system, you effectively have a pretty fast, if small,
> > interconnect. if your code can take advantage of that, then going
> > dual-core could well be a win. for instance, if your code is limited
> > by short-message, point-to-point latency, then increasing "SMP-ness"
> > should help a lot, especially if you are assuming mere gigabit.
> Well, actually I'm still not sure about this. The CPUs inside the node will
> communicate fast, but then the network will be a bottleneck?
I was careful in what I said, and perhaps not explicit enough. if your
code has a lot of short, p-p messages, then the MPI will avoid the use
of the nic on paths within the machine. (at least myrinet and quadrics do).
that's a significant win, since intra-node is .8us/800MBps for an
older opteron cluster I have, vs 3.5/240 for inter-node.
so if you're only scaling to 8x, and you use dual-dual nodes, half your
messages will be very fast. if the inter-node fabric is gigabit, that
means .8/800 vs 30/80.
so just looking at an 8p cluster, assuming only p-p messages uniformly
two 2x2's will see .8/800 on half of all messages, 30/80
on the rest or 15.4/440 aggregate.
four 2x1's will see .8/800 on a quarter, 30/80 on the rest,
this is pretty rough, of course: if you had the right patterns,
you could do better or worse. and if you use collectives, you'll
always be limited at least by inter-node performance.
regards, mark hahn.
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