[TUHS] Why did PDPs become so popular?

Larry McVoy lm at mcvoy.com
Sat Dec 30 02:38:32 AEST 2017


On Fri, Dec 29, 2017 at 04:04:01AM -0700, Kevin Bowling wrote:
> Alpha generally maintained integer/ALU and clockspeed leadership for
> most of the '90s
> http://www.cs.columbia.edu/~sedwards/classes/2012/3827-spring/advanced-arch-2011.pdf

Wow, that first graph is the most misleading graph on CPU performance
I've ever seen.  Ever.

So from 1993 to 2000 the only CPUs released were Alphas?

That era was when I was busy measuring performance across cpus and
operating systems and I don't ever remember any processor being a
factor of 2 better than its peers.  And maybe I missed it, I only 
owned a couple of alpha systems, but I never saw an Alpha that was
a game changer.  Alpha was cool but it was too little, too late to
save DEC.

In that time period, even more so now, you had to be 2x better to get
a customer to switch to your platform.

	2x cheaper
	2x faster
	2x more reliable

Do one of those and people would consider switching platforms.  Less than
that was really tough and it was always, so far as I remember, less than
that.  SMP might be an exception but we went through that whole learning
process of "well, we advertised symmetric but when we said that what we
really meant was you should lock your processes down to a processor
because caches turn out to matter".  So in theory, N processors were N
times faster than 1 but in practice not so much.

I was very involved in performance work and cpu architecture and I'd love
to be able to claim that we had a 2x faster CPU than someone else but we
didn't, not at Sun and not at SGI.

It sort of make sense that there weren't huge gaps, everyone was more or
less using the same sized transistors, the same dram, the same caches.
There were variations, Intel had/has the biggest and most advanced
foundries but IBM would push the state of the art, etc.  But I don't
remember anyone ever coming out with a chip that was 2x faster.  I
suspect you can find one where chip A is introduced at the end of chip
B's lifespan and A == 2*B but wait a few month's and B gets replaced 
and A == .9*C.

Can anyone point to a 2x faster than it's current peers chip introduction?
Am I just not remembering one or is that not a thing?

--lm


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