HP-MPI User's Guide (11th Edition)

Tuning
Processor subscription
Chapter 5 189
Processor subscription
Subscription refers to the match of processors and active processes on a
host. Table 5-1 lists possible subscription types.
When a host is over subscribed, application performance decreases
because of increased context switching.
Context switching can degrade application performance by slowing the
computation phase, increasing message latency, and lowering message
bandwidth. Simulations that use timing–sensitive algorithms can
produce unexpected or erroneous results when run on an over-subscribed
system.
In a situation where your system is oversubscribed but your MPI
application is not, you can use gang scheduling to improve performance.
Refer to “MP_GANG” on page 145 for details. This is only available on
HP-UX systems.
Table 5-1 Subscription types
Subscription type Description
Under subscribed More processors than active
processes
Fully subscribed Equal number of processors
and active processes
Over subscribed More active processes than
processors