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From: Brett <ggtgp@yahoo.com>
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Subject: Re: is Vax addressing sane today
Date: Wed, 11 Sep 2024 16:39:23 -0000 (UTC)
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Anton Ertl <anton@mips.complang.tuwien.ac.at> wrote:
> 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.

IBM had huge caches the PC’s could not match and smart IO processors to
handle much of the load, that PC’s had to handle with the CPU because they
were cheap.

You could go into this as my knowledge is mostly SWAG based off marketing
bull and what little I know.

Then there is the issue of cheap PC’s that fail, and a mainframes have a
higher level of redundancy and failover. Failed business transactions can
cost millions, more than the machine is worth, so saving pennies on
hardware is stupid.

> 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