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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: what's a mainframe, was is Vax addressing sane today
Date: Wed, 11 Sep 2024 16:21:19 -0000 (UTC)
Organization: Taughannock Networks
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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.

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.

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.

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.



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John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
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