Do You Know??

[Aromal]

Do you know what is 'PAGE THRASHING'?

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.