Virtual Memory Performance Implications
While virtual memory makes it possible for computers to more easily
handle larger and more complex applications, as with any powerful tool,
it comes at a price. The price in this case is one of performance — a
virtual memory operating system has a lot more to do than an operating
system that is not capable of virtual memory. This means that
performance will never be as good with virtual memory than with the same
application that is 100% memory-resident.
However, this is no reason to throw up one's hands and give up. The
benefits of virtual memory are too great to do that. And, with a bit of
effort, good performance is possible. The thing that must be done is to
look at the system resources that are impacted by heavy use of the
virtual memory subsystem.
Worst Case Performance Scenario
For a moment, take what you have read in this chapter, and
consider what system resources are used by extremely heavy page fault
and swapping activity:
RAM — It stands to reason that available RAM will be low
(otherwise there would be no need to page fault or swap).
Disk — While disk space would not be impacted, I/O bandwidth
would be.
CPU — The CPU will be expending cycles doing the necessary
processing to support memory management and setting up the
necessary I/O operations for paging and swapping.
The interrelated nature of these loads makes it easy to see how
resource shortages can lead to severe performance problems. All it
takes is:
A system with too little RAM
Heavy page fault activity
A system running near its limit in terms of CPU or disk I/O
At this point, the system will be thrashing, with performance
rapidly decreasing.
Best Case Performance Scenario
At best, system performance will present a minimal additional load
to a well-configured system:
RAM — Sufficient RAM for all working sets with enough
left over to handle any page faults[1]
Disk — Because of the limited page fault activity, disk I/O
bandwidth would be minimally impacted
CPU — The majority of CPU cycles will be dedicated to
actually running applications, instead of memory management
From this, the overall point to keep in mind is that the
performance impact of virtual memory is minimal when it is used as
little as possible. This means that the primary determinant of good
virtual memory subsystem performance is having enough RAM.
Next in line (but much lower in relative importance) are
sufficient disk I/O and CPU capacity. However, these resources only
help the system performance degrade more gracefully from have faulting
and swapping; they do little to help the virtual memory subsystem
performance (although they obviously can play a major role in overall
system performance).
