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From: neil@uninet.cpd.com (Neil Gorsuch)
Newsgroups: comp.sys.dec
Subject: Re: Why RISC doesn't support large nuber of users?
Message-ID: <1990Feb3.002004.6974@zardoz.cpd.com>
Date: 3 Feb 90 00:20:04 GMT
References: <25c936b0141@vms.huji.ac.il>
Sender: neil@uninet.cpd.com
Reply-To: neil@uninet.cpd.com (Neil Gorsuch)
Organization: Uninet Peripherals, Santa Ana, CA, USA
Lines: 62

In article <25c936b0141@vms.huji.ac.il> YEHAVI@vms.huji.ac.il writes:
>  I have a question which I am very curious to know the answer for:
>Take a VAX-6300 which is rated at 4MIPS and a RISC machine which is rated
>as over 14 MIPS. THe VAX 6300 can hold over 100 users (few are doing heavy
>jobs, most do light jobs) and a few batches. The RISC machine from the other
>side is rated by its manufacturer to 35 users... Altough there are a lot
>types of RISC machines I hear from most suppliers that RISC machine cannot
>hold a large number of users, but very few suppliers claim they can.
>  So, what is the problem with the RISC machines? Context switching,
>initiating a lot of I/O requests, or what? (I am talking on a mixed academic
>environement which includes compilations, statistics packages, databases,
>etc., little from everyhting).

There are a lot of reasons, but remember that the ratings are heavily
slanted towards computer manufacturers selling minicomputers and
servers rather than desktop workstations.  If it were just CISC versus
RISC, the Microvax III would be rated at 50 to 100 users according to
it's MIPS rating.

1. A lot of it is the infamous $50,000 card cage syndrome.  Computer
manufacturers have to very competitive with their desktop workstations
that have a great MIPS rating, but they always seem to charge a large
amount for card cages.

2. There is a certain amount of problems with context switching.  I
don't know about Dec RISC machines, but in the case of Suns, a SPARC
processor has a fixed number of register windows, which are very easy
to context switch between.  But I would much rather be one of 35
people using a 12 MIPS Sparcstation with 64 Mbytes of memory, assuming
that most were light usage, than one of 100 people using a 4 MIPS VAX
6300.

3. The larger machines have higher speed i/o busses, but a lot of disk
time is spent waiting for the head to get to the proper spot on the
disk, and you can buy 10.5 mS average seek time SCSI disks with over 1
Mbyte/second transfer rates.

4. The smaller machines do not usually have any means of adding
expansion cards for things like serial ports, and putting 50 people on
one ethernet leg would impact ethernet performance, but there are now
cheaper ethernet serial interfaces, and even a cheaper still SCSI
serial interface that won't degrade the ethernet.

All in all, I would say that a Decstation (assuming that you can add
enough memory) can handle at least half the users that a 6300 can.
And I doubt that putting 100 people on a 4 MIPS machine is ever going
to please the users of that machine, anyway.  And even if it's only a
third the users, the price difference is a lot more than 3 times!  If
you are stuck with giving people terminals, what I would do is use a
Decstation for every 16 users, and if you shop around for the
peripherals, you can buy a 16 user system, including Decstation,
disk(s), serial interfaces and terminals for about $25,000 total 8-).
And the users will be much more pleased with almost a MIP each, and
think of the money you will save on service contracts 8-).  Or if the
users are used to noticable key stroke response delays, put 32 people
on a Decstation for around $35,000.

--
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