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From: Stephen Fuld <sfuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: what's a mainframe, was is Vax addressing sane today
Date: Wed, 11 Sep 2024 10:18:44 -0700
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On 9/11/2024 9:21 AM, John Levine wrote:
 > According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:
 >> Lawrence D'Oliveiro <ldo@nz.invalid> writes:
 >>> On Tue, 10 Sep 2024 16:32:05 GMT, Anton Ertl wrote:
 >>>
 >>>> It seems that during the late 1990s, IBM was not particularly 
interested
 >>>> in mainframe per-CPU performance.
 >>>
 >>> Mainframes were never about CPU performance.
 >>
 >> The S/360 Model 91 and the Model 195 certainly were about the maximum
 >> CPU performance.  And I doubt that IBM would have spent all the effort
 >> with ECL and a superscalar OoO implementation for some of the ES/9000
 >> machines if CPU performance was considered unimportant at the time.
 >>
 >> It's an interesting question why they did not follow up their
 >> superscalar OoO ECL implementations with a superscalar OoO CMOS
 >> implementation in addition to the scalar in-order 9672. ...
 >
 > IBM definitely cared about maximum performance in the 1950s and early 
1960s.

Yes.  And remember, one of the goals of S/360 was to provide an 
architecture that could handle both scientific (i.e. compute bound) and 
business (i.e. I/O bound) workloads.


 > The goal of STRETCH was specifically to make the fastest possible 
computer. It sort of
 > succeeded, late and over budget and not as fast as they hoped, but 
still the fastest
 > computer in the world for a while. It was a success in that they 
reused a lot of the
 > technology like the fast core memory in later computers.
 >
 > The 360/91 was also intended to be the fastest possible computer, 
which again it sort of
 > was, late and over budget. One thing that STRETCH and the /91 shared 
was that they were
 > extremely complicated. STRETCH had variable sized bytes and and 
addressing modes that I
 > never entirely figured out. The /91 had an instruction queue with 
loop mode and out of
 > order operations and register renaming and imprecise interrupts. When 
the CDC 6600 came
 > out, a much simpler design from a tiny company that was nonetheless 
faster than the /91,
 > they knew they had a problem. The /95 and /195 were minor upgrades of 
the /91 but that was
 > the end of their supercomputer efforts.


Mostly true, except for the 3090 vector facility.


 > The point of a mainframe is balanced performance. The CPU of a 360/30 
was extremely slow
 > but it was fast enough to drive a disk or two and a printer and card 
read/punch and get a
 > lot of useful work done. Mainframes have had channels since the 709 
in the late 1950s so
 > they have a lot of I/O capacity.  Modern ones have terabytes of RAM 
and exabyte of disk.

Yes.


 > They also care deeply about reliability. Modern mainframes have 
multiple kinds of error
 > checking and standby CPUs that can take over from a failed CPU, 
restart a failed
 > instruction, and the program doesn't notice.  I think you'll find a 
pattern since the
 > CDC shock of making CPUs fast enough to keep the RAM and I/O devices 
busy while having
 > the error checking and recovery features so the systems keep running 
for years at a time.

Yes, but they also have to keep producing faster and faster CPUs so they 
can entice current customers to upgrade and thus meet their revenue goals.



-- 
  - Stephen Fuld
(e-mail address disguised to prevent spam)