[comp.os.vms] Considerations For Trading A 750 For A 780...

CLAYTON@XRT.UPENN.EDU ("Clayton, Paul D.") (09/10/87)

Information From TSO Financial - The Saga Continues...
Chapter 21 - September 9, 1987

Holt Farley from Columbia Univ. Civil Engineering has asked the following 
question.

	We are considering replacing our VAX 11/750 CPU with a 11/780 which we 
can get VERY cheaply from another department here at Columbia Univ.  This 
machine will run VMS, and is used to support faculity/graduate research in an 
engineering department.  We do quite a bit of program development as well as 
run many programs that use a lot of system resources:  finite element routines 
(large matrices), symbolic languages (MACSYMA, SMP), TeX, etc.

My question is:    What kind of performance increase will we likely see
		   if we do purchase the 780?

	Obviously, the answer to this question depends upon the exact 
configuration of the computers and the work done.  Our 750 currently has 8Mb
of memory and can only support 10-12 typical users before response time gets
really terrible.  NULL mode is extremely rare!  I expect one of the main 
advantages to the 780 is that I can install lots of memory --- I plan to put
at least 16-20 Mb in the 780.

RESPONSE:

I have read various articles about MIP ratings between the various CPU types and
the bottom line is that the 750 and 780 and not that far apart, maybe less then
.2 MIPS. This is not alot by itself. Both systems can have a UNIBUS on them and
they will handle the same amount of traffic, give or take a little. The next
major difference is the CMI interface on the 750 versus the MASSBUS on the
780/785/86. The CMI is a VERY clean and efficient means of passing information
and from the 750 system I used to run speed tests on, it came close to the
MASSBUS throughput. 

The BIGGEST advantage is MEMORY, and you said that 16-20 MB is what you are
shooting for. I would not stop at 20MB. The 780 memory is VERY CHEAP and 
available. I just bought some 785 memory that with a trade in allowance on
the original 1MB boards it coast me $850 per 4MB board. As a school you could 
probably get it cheaper as a right off for some system clearing house. I would
go for as much memory as possible and cut down on PAGING and SWAPPING. The 
extra .2 MIPS may make a BIG difference as long as it is NOT spent in system
space doing paging and swapping. Adjustments of working set values in the UAF
records would also be advisable to make the best use of the additional memory.
I would also recommend the addition of the Floating Point Accelator card set
to help with the number crunching. It also helps to a certain extent in VMS 
doing other tasks.

Bottom line. Get the 780, with the MOST memory possible and consider adding
the FPA to the processor.

Good luck.. :-)

Paul D. Clayton - Manager Of Systems
TSO Financial - Horsham, Pa. USA
Address - CLAYTON%XRT@CIS.UPENN.EDU