now many nodes can a lan support?

Jim Lux James.P.Lux at
Fri Jan 10 10:28:11 PST 2003

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 

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?
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