now many nodes can a lan support?

Mike Eggleston mikee at mikee.ath.cx
Fri Jan 10 13:43:09 EST 2003


On Fri, 10 Jan 2003, Jim Lux wrote:

> It's basically a communications bandwidth problem...
> 
> What's your bottleneck?  I would assume it's the link to the head in your 
> case, since all those 1KB packets heading out and 100B packets heading back 
> have to go through it.
> 
> 1KB > roughly 10 kbits (by the time you add headers, etc.)
> 100B > roughly 1 kbit..
> Clearly the "head to node" traffic will dominate the link, it's ten times 
> bigger.
> 
> Figure that 10 Mbps wire can carry, comfortably, 5 Mbps..
> 
> 5000 kbps/10 kb = 500 packets/second
> 
> 
> Now you need to find out how long it takes for a node to process the 
> packet..  If it takes 1 second, you can support 500 nodes. If it takes 0.01 
> seconds you can support 5.
> 
> 
> This is a very rough and ready estimate, of course...  You need to address 
> such issues as:
> 
> 1) synchronization of the nodes... if they all try to talk at the same 
> time, your throughput will drop (a lot). If node 2 tries to talk to the 
> head at the same time node 1 is, then either node 2 blocks, or its packet 
> gets held somewhere.  In your application, the dominant traffic appears to 
> be from the head to the nodes, so it would inherently tend to be sequenced 
> and non-simultaneous.
> 2) variability of the processing time on the node...
> 
> This is all pretty standard queuing theory stuff.  You could write a little 
> simulation program to try it all out.  R.G.Brown's book at the Duke Brahma 
> site talks quite a bit about how to calculate parallelism and speedup
> 
> 
> 
> 
> At 11:15 AM 1/10/2003 -0600, you wrote:
> >My guess is this question has been asked before, but I've not been
> >able to find it in the archive file. The question is given a typical
> >10Mb/s lan how many nodes can a cluster support? Assume the cluster
> >has its own switch, the head and nodes are connected in a star with
> >the switch, the cluster lan is isolated from all other non-cluster
> >network traffic, the only way to reach a node is through the head,
> >ignore extra traffic from TCP handshakes and such, and the the
> >data packet for a work unit is 1KB with a 100B results packet back
> >to the head.
> >
> >How do I calculate this?
> >
> >Mike
> >_______________________________________________
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> 

Wonderful information and examples. I was worried that I would not be
able to support 20+ nodes with my current equipment and the speed
of node processing.

Mike
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