[rescue] Dual Processors and applications

[Patrick Giagnocavo]

On Tue, May 07, 2002 at 12:30:18PM -0500, Brian Dunbar wrote:
> Scenario:
> A dual processor Unix box (sun or hp).  An application (CAD tool) that was
> not specifically optimized for dual processors.  Will the app take advantage
> of the dual processors?

Others will chime in, but here is the received wisdom as I know it:

1.  If there are threads in the application, it will derive benefits
as long as the threading model on that architecture can run different
threads on different CPUs.

2. If single-threaded (ie not threaded) it may receive a boost only to
the extent that other operations occurring on the box will be handled
by the other CPU, such as receiving email, running Netscape, etc.  

If many people are logging in to the machine their smaller tasks will
be assigned to the second CPU.  This may or may not be significant in
terms of increasing performance.

A tradeoff is that memory accesses may be slower due to 2 CPUs sharing
the RAM, which may or may not offset any gain (again, this is
dependant on the specific memory subsystem that the motherboard is
using).

3.  Multiple copies of the same app will see a boost, as each separate
process can be run on either CPU.  Expect about a 1.8 times
performance increase for 2 instances of the app running on a dual CPU
box vs. 2 apps on a single CPU (assuming that CPU power is a big
factor in the speed of the app).

I think that the book "Computer Architecture: A Quantitative Approach"
by John Hennessy goes into this sort of thing in substantial detail.

Cordially

Patrick Giagnocavo
patrick at zill.net