HP Vectra XU 6/XXX Optimization Manual page 10

This distinction is important. If you are running a multithreaded operating system, and use
multithreaded applications or multiple single-threaded applications, installing a second Pentium
Pro processor will significantly improve the performance of your PC. If, however, you run just
one single-threaded application, then installing a second processor will offer little, or no
performance advantage.
The following table summarizes the benefits of installing a second Pentium Pro processor
according to the operating system and applications you use.
Single-threaded
operating system
Multithreaded
operating system
Other Benefits of Dual Processing
One of the most important functions that a processor performs is handling interrupts. An
interrupt is a request for attention or service from another area of the PC. The processor's
ability to respond to interrupts is an essential part of your PC's operation. Interrupts allow your
PC to respond to unexpected events.
To handle an interrupt, your processor must first suspend its current activity. This means that
your PC's operation will be slowed temporarily while the interrupt is being handled.
With two Pentium Pro processors installed the responsibility of handling interrupts is shared
between them. When an interrupt request is received, the two processors arbitrate between
themselves to see which processor is performing the lowest priority work. The selected
processor will be allocated to handle the interrupt, leaving the other processor free to continue
with its work.
Multiple applications
None
Performance improvement for all
applications (even single-threaded)
One multithreaded application
None
Greatest benefit from multiprocessing