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.
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.
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”).
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”).
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.
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.
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.