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. 

The Windows Managed Swap File Grows and Shrinks

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. 

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. 

But What Settings Should I 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. (Can you say "crash"?). 
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 if something screwy 
happens. Again, System Monitor is an excellent tool to use in making this 
decision. 

Where is My Swap File?

In Windows 95/98, the swap file is Win386.swp and is located in the Windows 
directory if you are using a Windows managed swap file. If you are 
specifying your own settings, it will be in the root directory of the drive 
you chose to put it on. In Windows NT, the swap file is called 
pagefile.sys. In Windows 3.1, if it is a dynamic swap file it will be 
called win386.swp and if it is a static swap file it will be called 
386spart.par and be located in the root directory of the drive that windows 
3.1 is installed on. 
System.ini - The Real Swap File Settings
The settings for the swap file are actually under the [386Enh] section in 
the System.ini file. You can actually override the swap file settings here, 
if you prefer. Here is a typical [386Enh] section, note the last three 
lines pertaining to the swap file. Note that the values for min and max are 
in KILOBYTES in the system.ini file. (not Mb like the setting in system 
properties) 

[386Enh]
ebios=*ebios
woafont=dosapp.fon
mouse=*vmouse
device=*dynapage
device=*vcd
device=*vpd
device=*int13
keyboard=*vkd
display=*vdd,*vflatd
PagingDrive=C:
MinPagingFileSize=65536
MaxPagingFileSize=395264

If you decide to edit the system.ini file, be sure to use upper and lower 
case exactly as it appears. It matters. For example, pagingdrive=c: is NOT 
the same as PagingDrive=C: (in fact, there is an issue in Win98 Second 
Edition where the computer hangs if a lower case drive letter is specified 
for the paging file location...but that's another story). The settings 
shown here are for a swap file on drive C: with a minimum size of 64 MB and 
Maximum size of 386 MB. (multiply each MB value by 1024 to get KB) 

Conclusion

You can improve performance by specifying your own Swap File settings. This 
is particularly significant if you have less than 64 MB of RAM and Windows 
is using your swap file frequently. If, however, you have a nice fast hard 
drive and a large amount of RAM (like 128 MB or 256 MB) then little will be 
gained by changing the swap file settings from Windows managed. If your 
swap file is rarely used, then the fact that it is of variable size and 
fragmented will not degrade your computer's performance appreciably. 

A little tip for good measure: 
If you ever copy a hard drive's contents or the directory that contains the 
swapfile using Windows Explorer, be sure NOT to copy the swapfile. If you 
do, most likely Windows will crash. 


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