Embedded Systems Question:

Some operating systems (such as UNIX or Windows in enhanced
mode) use virtual memory. Virtual memory is a technique for
making a machine behave as if it had more memory than it
really has, by using disk space to simulate RAM (random-
access memory). In the 80386 and higher Intel CPU chips,
and in most other modern microprocessors (such as the
Motorola 68030, Sparc, and Power PC), exists a piece of
hardware called the Memory Management Unit, or MMU.
The MMU treats memory as if it were composed of a series
of “pages.” A page of memory is a block of
contiguous bytes of a certain size, usually 4096 or 8192
bytes. The operating system sets up and maintains a table
for each running program called the Process Memory Map, or
PMM. This is a table of all the pages of memory that
program can access and where each is really located.
Every time your program accesses any portion of memory, the
address (called a “virtual address”) is processed by the
MMU. The MMU looks in the PMM to find out where the memory
is really located (called the “physical address”). The
physical address can be any location in memory or on disk
that the operating system has assigned for it. If the
location the program wants to access is on disk, the page
containing it must be read from disk into memory, and the
PMM must be updated to reflect this action (this is called
a “page fault”).

Because accessing the disk is so much slower than
accessing RAM, the operating system tries to keep as much
of the virtual memory as possible in RAM. If you’re running
a large enough program (or several small programs at once),
there might not be enough RAM to hold all the memory used
by the programs, so some of it must be moved out of RAM and
onto disk (this action is called “paging out”).
The operating system tries to guess which areas of memory
aren’t likely to be used for a while (usually based on how
the memory has been used in the past). If it guesses wrong,
or if your programs are accessing lots of memory in lots of
places, many page faults will occur in order to read in the
pages that were paged out. Because all of RAM is being
used, for each page read in to be accessed, another page
must be paged out. This can lead to more page faults,
because now a different page of memory has been moved to
disk.

The problem of many page faults occurring in a short time,
called “page thrashing,” can drastically cut the
performance of a system. Programs that frequently access
many widely separated locations in memory are more likely
to cause page thrashing on a system. So is running many
small programs that all continue to run even when you are
not actively using them. To reduce page thrashing, you can
run fewer programs simultaneously. Or you can try changing
the way a large program works to maximize the capability of
the operating system to guess which pages won’t be needed.
You can achieve this effect by caching values or changing
lookup algorithms in large data structures, or sometimes by
changing to a memory allocation library which provides an
implementation of malloc() that allocates memory more
efficiently. Finally, you might consider adding more RAM to
the system to reduce the need to page out.

Page thrashing causes performance degradation. You can
increase the ram disk size to reduce page thrashing.