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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: is Vax addressing sane today
Date: Wed, 11 Sep 2024 09:32:04 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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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.  Here are
three speculations of what happened:

1) They had such a project and it did not work out, and the "never
about CPU performance" spin is a sour-grapes type rationalization of
the result.

2) They expected their mainframe market to be eaten up by the Unix
and/or WNT markets, and did not want to invest a lot into the
development of mainframe CPUs.  Again, the "never about CPU
performance" spin is a sour-grapes type rationalization of the result.

3) They had decided that they had a captive market in the mainframes,
with software that was written for lower-powered CPUs, that the rapid
CMOS advances in the 1990s would give them enough of a performance
push to satisfy the needs of this software, so no more sophisticated
CPU designs that the 9672 was necessary (and the G5 and G6 of the 9672
indeed gave them more CPU power than ever).  The "never about CPU
performance" reflected their position at the time and also served to
placate anyone who pointed out that the per-CPU performance was
inferior to that of other CPUs of the time, including IBM's own
RS/6000 line.

Eventually they seem to have decided that per-CPU performance is
important after all, with the superscalar z990 in 2003 and the OoO
z196 in 2010.  But of course Dennart scaling was slowing down around
2003, so they needed to increase IPC to increase per-CPU performance.
And even if they don't need more per-CPU performance than other
architectures, they apparently do need advances over earlier
generations of their own machines and maybe to discourage competition
from emulators or startups.

>They were about high I/O 
>throughput for efficient batch operations.

Batch operations?  I wonder how much CPU time on mainframes in the
1990s and today is spent on that compared to interactive applications
such as online transaction processing.

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>