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Chapter 4. Physical and Virtual Memory

Writing modified pages to a dedicated area on a mass storage device (usually known as swapping

or paging space)

Marking unmodified pages as being free (there is no need to write these pages out to disk as they

have not changed)

In order to determine appropriate working sets for all processes, the operating system must track

usage information for all pages. In this way, the operating system can determine which pages are

being actively used (and must remain memory resident) and which pages are not (and therefore, can

be removed from memory). In most cases, some sort of least recently used algorithm determines

which pages are eligible for removal from process working sets.

4.4.3. Swapping

While swapping (writing modified pages out to the system swap space) is a normal part of a sys 

tem's operation, it is possible to experience too much swapping. The reason to be wary of excessive

swapping is that the following situation can easily occur, over and over again:

Pages from a process are swapped

The process becomes runnable and attempts to access a swapped page

The page is faulted back into memory (most likely forcing some other processes' pages to be

swapped out)

A short time later, the page is swapped out again

If this sequence of events is widespread, it is known as thrashing and is indicative of insufficient

RAM for the present workload. Thrashing is extremely detrimental to system performance, as the

CPU and I/O loads that can be generated in such a situation can quickly outweigh the load imposed

by a system's real work. In extreme cases, the system may actually do no useful work, spending all its

resources moving pages to and from memory.

4.5. 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 is never as good with virtual memory as it is when

the same application 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.

4.5.1. 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 is low (otherwise there would be no need to page

fault or swap).

Disk   While disk space would not be impacted, I/O bandwidth (due to heavy paging and swap 

ping) would be.