Re: [PLUG] Looking to buy new dual-processor system

["William H. Magill" <magill@isc.upenn.edu>]

>   Of course, the real question is... why are you so interested in a dual
>   processor machine?  What do you plan on doing with it? 

This is a very important question. 
You may have very unrealistic expectations.

Does anything you are doing really support multiple processors anyway?

SMP is an old technology that nobody seems to get right...still.
SMP (many hammers) is not the equivilant of  having "a bigger hammer." 

1 processor  gets you 1 Processor worth of performance
2 processors gets you about 1.8 processors (or less), on a good day. 
3 processors gets you around 2.5 processors
4 processors gets you around 3.1 

But don't forget, these numbers come from Applications that know about
SMP. Something like 90% of applications will never have any possible reason
to use the capabilities of SMP. So you are left with the fact that the OS
probably is running two separate applications at more or less the same time
- one on each processor. 

The same is true of clustering.

Are you looking for redundancy, ie fault tolerance? Most boxes won't run
with EITHER CPU, but will run with one CPU, only if CPU 0 is the one
running. 

Beyond 4, the performance numbers go downhill fast. By the time you are
talking about 8 cpus in a  box, you're only seeing about 5x performance.

The 1999 OS relases seem to have moved those numbers up slightly, but in
general anything beyond 4 cpus in a box is still a waste of money.

That's one of the reasons that the new Compaq Alphas (Wildfire) boxes are
built on "quads" - 4 processor building blocks - and massive backplane
switches. 

If you are doing anything with Photoshop or similar graphic or linear
editing tools, then multiple processors or clusters become a different
story. Photoshop itself as well as most of its friends have been optomized
for multiple processor functionality -- the kind of crunching done in a
"render" is serious fodder for all kinds of hardware parallelism and the 
like. This is one of the reasons that Apple has their dual processor 500mhz
PowerPC box and why the G4 has the graphics acceleration instructions.
(These have nothing to do with the display, just rendering.)

Time sharing can benefit from multiple processors IF things like I/O and
the like are NOT funneled through one CPU because it owns the driver.

This is a big topic, but a fun one. Lots of HPC (High Performance
computing) folks out there are currently slobbering all over Compaq's Alpha 
GS (Wildfire) series on just these issues.
-- 
                        www.tru64unix.compaq.com
                              www.tru64.org
                             comp.unix.tru64
                        
T.T.F.N.
William H. Magill                          Senior Systems Administrator
Information Services and Computing (ISC)   University of Pennsylvania
Internet: magill@isc.upenn.edu             magill@acm.org
http://www.isc-net.upenn.edu/~magill/


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