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Managing the Windows Swap File
What is the Swap File?
To execute a program in Windows, it first needs to be loaded
into memory (RAM). Windows lets you run multiple programs simultaneously
and chances are that they won't all fit into memory at the same
time. For that purpose, Windows uses what is called Virtual Memory
to simulate RAM, pretending it has more memory than what is actually
build into the PC. It does this by moving data from real memory
to a special file on the hard drive, called the swap file in Windows
95/98 or page file in Windows NT. This, in effect, allows Windows
to address more memory than the amount of physical RAM installed.
Without it, we would not be able to run windows on machines with
limited RAM. For example, think back to when Windows 95 first
came out, the average computer had 8 to 16 Mb of Ram. It would
not have been possible to run Win95 and applications without using
virtual memory. Program code and data are moved in pages (memory
allocated in 4K or 16K segments within a 64K page frame) from
physical memory to the swap file. As the information is needed
by a process, it is paged back into physical memory on demand
and, if necessary, windows may page other code or data to the
swap file in its place.
Swap-File size changes
By default, windows manages the swap file dynamically, meaning
that Windows increases and decreases its size as needed. For most
users it is recommended to let Windows manage it to ensure that
there is always sufficient memory addressing for applications
and processes. The advantage of a dynamic swap file is that it
does not permanently make hard disk space unavailable, it can
only grow if there is sufficient disk space. When you shut down,
the swap file is being deleted. It is re-created again when Windows
is started. The disadvantage of a Windows managed swap file is
that it becomes fragmented, and performance can be lost due to
frequent resizing as your computer is performing memory intensive
tasks. Fortunately, educated users of Windows can specify their
own virtual memory settings if they deem it necessary.
How To Specify Your Own Virtual Memory Settings
You access these settings from within the System Control Panel
applet. The easiest way to get to that from the Desktop is to
right-click on the My Computer icon and choose Properties from
the menu. Another way is to click Start/Settings/Control Panel/System.
Click the Performance tab and click the Virtual Memory button.
If you see this for the first time, the option Let Windows Manage
my virtual memory is probably selected and the other fields are
grayed out. Click the Let me specify my own... and choose the
drive you want to put your swap file on by clicking the drop list
in the Hard Disk field and state the minimum and maximum swap
file size in the appropriate fields.
What Settings To Specify?
Most importantly, you do not want Windows to run out of memory,
so make sure that the size you specify will be sufficient. Base
your decision on how much RAM you have, and how demanding your
applications are. A good way to get a handle on this is to monitor
free physical memory, swap file size, and swap file in use. Install
System Monitor from Add/Remove Programs in Control Panel. Click
the Windows Setup tab and highlight the System Tools category
and click Details. Put a check in System Monitor and click OK
and then OK again. You will be prompted to insert your Windows
CD. After you get System Monitor installed, open it by going to
Start/Programs/Accessories/System Tools and go to Edit and choose
Add Item. Click the Memory Manager Category and choose the items
above. Use this utility for a while to monitor what Windows is
doing while you run your applications.
Choosing the Drive
If you only have one hard drive, obviously there won't be another
one in the list to choose from. In general, choose the fastest
drive for your swap file. If you have a second hard drive, and
it is on the Second IDE controller (Secondary Master or Secondary
Slave) you should choose that drive for your swap file to reside
on. The reason for this is, modern Dual PCI IDE controllers on
Pentium motherboards allow simultaneous access to two drives on
separate IDE channels. The technical term for this is Peer Concurrency.
Drives on the same IDE channel cannot be accessed simultaneously
(and also, the read/write heads on a single drive can't be in
two places at once). The benefits of having your swap file on
a drive on the second IDE controller should be obvious: Windows
will be able to work from the C: drive AND page to and from the
swap file on the other drive at the same time! If your system
uses the swap file often, this will increase performance significantly.
Setting the Swap File Size
There are advantages to setting a fixed swap file size (that
is, equal minimum and maximum settings). The swap file will be
a static size, which means that it can occupy contiguous cluster
chains and not become fragmented. Another advantage is that since
it is always the same size, Windows doesn't have to take the time
or resources to resize it. This will improve access time. The
disadvantage of a fixed size swap file is that it cannot grow
beyond the maximum if the need arises. If you want to have the
best of both worlds, a swap file that doesn't grow and remains
contiguous, yet can be increased if you inadvertently bite off
more than you can chew performing some memory intensive graphics
editing operation or something, then choose a minimum size that
is large enough that it will never need to be increased. For example,
if you set a minimum of 128 Mb and a maximum of 500 Mb, you should
never see the swap file size increase, but more will be available
to Windows should something bad happen. Again, System Monitor
is an excellent tool to use in making this decision.
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