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From: Lynn Wheeler <lynn@garlic.com>
Newsgroups: comp.arch
Subject: Re: what's a mainframe, was is Vax addressing sane today
Date: Fri, 13 Sep 2024 08:45:46 -1000
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John Levine <johnl@taugh.com> writes:
> 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.

shortly after joining IBM, I got pulled into effort to multithread
370/195 ... 195 didn't have branch prediction or speculative execution
so conditional branches drained pipeline and most codes ran at half
rated throughput. Two (simulated, "red/black") instruction streams
running at half speed would achieve rated throughput.

They also claimed that the main difference between 360/195 and 370/195
was introduction of ("370") hardware retry (masking all sorts of
transient hardware errors). Some vague recall mention that 360/195 mean
time between some hardware check was three hrs (combination of number of
circuits and how fast they were running).

Then decision was made to add virtual memory to all 370s and it was
decided that the difficulty in adding virtual memory to 370/195 wasn't
justified ... and all new work on machine was dropped.

Account of end of ACS/360 ... Amdahl had won the battle to make ACS, 360
compatible ... but folklore is then executives were afraid that it would
advance state-of-the-art too fast and IBM would loose control of the
market ... includes some references to multithreading
patents&disclosures.
https://people.computing.clemson.edu/~mark/acs_end.html

also mentions some of ACS/360 features show up more than 20yrs later
with ES/9000.

-- 
virtualization experience starting Jan1968, online at home since Mar1970