From: cy@solbourne.com (Cy Foughty)
Subject: Re: Xterminal-Server ratio wanted
Organization: Solbourne Computer, Inc., Longmont, CO
Date: Tue, 15 Sep 1992 22:12:30 GMT

In article <PD.92Sep15183212@herts.x.co.uk> pd@x.co.uk (Paul Davey) writes:
>
>One of our distributors was telling me the other week of a pair of
>sites using multiple processor Data General machines.
>
>They are running 100 - 150 X terminals per box with no problems.
>
>Software is mixed office automation stuff with X.desktop.
>
>The DG hosts have 96 or 128Mb RAM mind you.
>
>Response is apparently excellent.
> Deleted junk.....

I can believe that. The question that needs to be asked is "how many 
ethernet segments are used to support those 100 Xterminals?".

I have performed some tests using a network monitor that also can 
generate traffic. The rules I came up with usually give a good response
time on Xterminals.

			Xterminal Configuration Rules

1. Separate ethernet segments for Xterminals and ONLY Xterminals!!!!!!
   Ratio: 1 ethernet segment for
   every 100 Xterminals with no more than 40 to 50 Xterminals having 
   heavy graphics programs running. If ALL of your Xterminals are running
   a lot graphics then reduce the ratio to 1 enet segment for every 30-50
   Xterminals. You'll need to experiment. 

2. Because the X protocol trys to use commands instead of transfering images
   use RISC based Xterminals such as the NCD 19c (19" color). The draw functions
   will be performed faster. This is one of the most important rules!!!!!

3. When setting up the Xterminals DO NOT USE NFS for anything!! NFS is a pig!!
   Use the tftp protocol!

4. Xterminals require SYMMETRIC MULTIPROCESSOR SERVERS for good response time.
   A SparcStation 2 or and HP 7xx are not good Xterminal servers. I/O on the
   server side must be fast. Also configure the SYMMETRIC MULTIPROCESSOR SERVER
   with enough memory and cpu's. This can only be determined by analysis of the
   applications running on the server. Suffice to say uniprocessor boxes and
   ASYMMETRICAL MULTIPROCESSOR SERVERS (can you say "SUN") are the worst for 
   Xterminals.

5. Broadcast packets will kill the network. Xterminals seem to show it more.
   Keep the segments free from broadcast packets. Like, xdm access should be
   direct rather than through broadcast packets. Use routers to clean up the
   segment if needed.

Comments:

   I have used workstations for years and also Xterminals. There are cases
   where workstations are the solution. However, Xterminals provide great 
   service and you have access to the entire server resources. This is good
   only when the server is a SYMMETRIC MULTIPROCESSOR SERVER that has the
   memory, cpu's, and I/O controllers on separate boards. Because if the 
   server goes down, all that is needed is to pull the failing board and
   reboot the server. Performance might decrease while awaiting the new
   board, but the system will be running. 

   While developing software on a VAX with several hundred other engineers
   the performance went in the gutter. Compiles took hours instead of minutes.
   The solution the MIS department came up with: buy a bigger VAX. OK, fine.
   for about 6 months, then the same thing again! Same solution! The VAX 
   upgrades were in the millions of dollars!! The UNI-processor VAXes
   with I/O to multiple processes chokes. There is a bottleneck, it is
   called I/O!

   With SYMMETRIC MULTIPROCESSOR SERVERS that have multiple I/O channels
   (not all do) most of the bottlenecks vanish. 


   Xterminals work great if setup correctly with the correct server!!!!!!


-- 
        Cy Foughty                Solbourne Computer 
	Work: (214) 450-5993      15851 North Dallas Parkway Suite 675
	La Casa: (214) 578-8837   Dallas, TX 75248
	ARETC: Association for the Revival of Electron Tube Computing