First, we must introduce a new concept: virtual address
space. Virtual address space is the maximum amount of
address space available to an application. The virtual address space
varies according to the system's architecture and operating system.
Virtual address space depends on the architecture because it is the
architecture that defines how many bits are available for addressing
purposes. Virtual address space also depends on the operating system
because the manner in which the operating system was implemented may
introduce additional limits over and above those imposed by the
architecture.
The word "virtual" in virtual address space means this is the total
number of uniquely-addressable memory locations available to an
application, but not the amount of physical memory
either installed in the system, or dedicated to the application at any
given time.
In the case of our example application, its virtual address space is
15000 bytes.
To implement virtual memory, it is necessary for the computer system
to have special memory management hardware. This hardware is often
known as an MMU (Memory Management Unit).
Without an MMU, when the CPU accesses RAM, the actual RAM locations
never change — memory address 123 is always the same physical
location within RAM.
However, with an MMU, memory addresses go through a translation step
prior to each memory access. This means that memory address 123 might
be directed to physical address 82043 at one time, and physical address
20468 another time. As it turns out, the overhead of individually
tracking the virtual to physical translations for billions of bytes of
memory would be too great. Instead, the MMU divides RAM into
pages — contiguous sections of memory of a
set size that are handled by the MMU as single entities.
Keeping track of these pages and their address translations might
sound like an unnecessary and confusing additional step. However, it is
crucial to implementing virtual memory. For that reason, consider the
following point.
Taking our hypothetical application with the 15000 byte virtual
address space, assume that the application's first instruction accesses
data stored at address 12374. However, also assume that our computer
only has 12288 bytes of physical RAM. What happens when the CPU
attempts to access address 12374?
What happens is known as a page fault.
A page fault is the sequence of events occurring when a program
attempts to access data (or code) that is in its address space, but is
not currently located in the system's RAM. The operating system must
handle page faults by somehow making the accessed data memory
resident, allowing the program to continue operation as if the page
fault had never occurred.
In the case of our hypothetical application, the CPU first
presents the desired address (12374) to the MMU. However, the MMU has
no translation for this address. So, it interrupts the CPU and causes
software, known as a page fault handler, to be executed. The page
fault handler then determines what must be done to resolve this page
fault. It can:
Find where the desired page resides on disk and read it in
(this is normally the case if the page fault is for a page of
code)
Determine that the desired page is already in RAM (but not
allocated to the current process) and reconfigure the MMU to point
to it
Point to a special page containing only zeros, and allocate a
new page for the process only if the process ever attempts to
write to the special page (this is called a copy on
write page, and is often used for pages containing
zero-initialized data)
Get the desired page from somewhere else (which is discussed
in more detail later)
While the first three actions are relatively straightforward, the
last one is not. For that, we need to cover some additional
topics.
The group of physical memory pages currently dedicated to a
specific process is known as the working set
for that process. The number of pages in the working set can grow and
shrink, depending on the overall availability of pages on a
system-wide basis.
The working set grows as a process page faults. The working set
shrinks as fewer and fewer free pages exist. To keep from running out
of memory completely, pages must be removed from process's working
sets and turned into free pages, available for later use. The
operating system shrinks processes' working sets by:
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)
To determine appropriate working sets for all processes, the
operating system must track usage information for all pages. In this
way, the operating system determines which pages are actively being
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.
While swapping (writing modified pages out to the system swap
space) is a normal part of a system'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 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.
