Subject: Re: VAX 6460 being slow, IO bottlenecks and SMP woes ...
To: Eric Smith <>
From: John Klos <>
List: port-vax
Date: 03/21/2002 03:37:30
> > It wasn't. It does thing that today's desktops can't do. When is the
> > last time you got 99.999% uptime out of a PC?
> Not counting an eight-hour power failure, one of my PC-based servers
> has had under two minutes of unscheduled downtime in seven years of
> 24x7 operation.  That's better than 99.9999%.

But the point is that you cannot count on it. If PC hardware works for
seven years without fail, that is definitely the exception and not the
rule. PC hardware is generally mediocre. Even my Amiga hardware screams
quality next to PC hardware.

> > Even though the
> > theoretical  bandwidth of the PCI bus was 120+ MB/sec systems rarely
> > achieved > 1 - 2  MB/sec sustained throughput.
> I don't know what systems those were, but my first 486 PCI system
> routinely got sustained PCI throughput of over 15 MB/second, between
> disk, ethernet, and display.

The key word above: sustained. Sustained throughput is the amount of data
continuously moving through the bus. Video obviously doesn't count (we are
talking about servers here, aren't we?)

> Even many ISA bus systems could sustain over 4 MB/second.

Let's see... 16 bits, 2 clocks per bus cycle, 8 MHz... that makes for a
theoretical speed of 8 MB/sec. That's the theoretical maximum. Because of
the piss poor design of the ISA bus (it was poor even in the '80s), the
CPU cannot do anything else while bus transfers are taking place (64k DMA
is next to useless), meaning that any system, even in a perfect world,
that was doing 4 MB/sec would have exactly half of all CPU time be wasted
before any real work is done.

So, tell me - what, if any, 486 system ever existed that could do some
meaningful work while transferring 4 MB/sec and operating with half of
its CPU?

Again, the key word is sustain. The context is a discussion about servers,
so we're not talking about a synthetic benchmark and saying, "Yes, this
ISA SCSI controller can sustain 4 meg/sec"; we're talking about what a
server can sustain in the course of its work.

People like VAX because it is kick-ass hardware. It is engineering to an
extreme. VAX are reliable, and PCs are not (yes, that's a generalisation).
If you want to compare PCs and VAX, yet you wish to ignore those things
that make the VAX unique by dismissing them with one-off examples, then
you're in the wrong place.

John Klos