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Q83968: How Windows Determines the Recommended Maximum Swap File Size

Article: Q83968
Product(s): Microsoft Windows 95.x Retail Product
Version(s): 3.1,3.11
Operating System(s): 
Keyword(s): win31
Last Modified: 11-FEB-2002

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The information in this article applies to:

- Microsoft Windows versions 3.1, 3.11 
- Microsoft Windows for Workgroups versions 3.1, 3.11 
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3.10 3.11

WINDOWS

kbusage

SUMMARY
=======

This article discusses how Microsoft Windows version 3.1 determines the file
size to recommend for swap files.

MORE INFORMATION
================

Temporary Swap File
-------------------

- Q. What is the temporary swap file recommended maximum size?

  A. It is four times the physical RAM in the computer, rounded to the next 4
  megabytes (MB). For example, if you have 3 MB of free XMS memory, you can
  create 16 MB of virtual memory ([3 MB * 4] + 4MB = 16 MB). Four is the
  default value for the SYSTEM.INI [386Enh] switch "PageOverCommit=".


  The recommended maximum size is also limited by the "50 percent free hard
  drive space" rule (that is, a swap file cannot take more than 50 percent of
  the available, usable hard drive space [compressed or stacked hard drive
  space is not considered usable]).

  Whichever limit is reached first establishes the maximum size.

- Q. When I set a temporary swap file to a size larger than the maximum
  recommended size, the following error message is displayed. What does this
  message mean?

  Windows will not use more than the virtual memory specified by the
  Recommended Size. Are you sure you want to create a larger swap file?

  A. WIN386.EXE creates the temporary swap file at run time, and the file grows
  as necessary; the swap-file functionality only controls the maximum size
  allowable. You can never exceed 50 percent of the free hard disk space or
  four times the amount of physical RAM.

Permanent Swap File
-------------------

- Q. When I set a permanent swap file to a size larger than the recommended
  maximum size, Windows forces the setting back to the recommended maximum
  size, even when plenty of free hard drive space exists. When this occurs, the
  following message is displayed. What does this message mean?

  The number you specified is too large. The number currently displayed is the
  largest you can specify.

  A. When Windows sets up a permanent swap file, free and contiguous hard drive
  space limits the recommended maximum size. A permanent swap file must be
  located on unfragmented space; therefore, even if there is plenty of disk
  space, the swap file is limited by file fragmentation.

If you have an disk-optimization program, such as Speedisk by Norton Utilities or
Compress by PC Tools, you can use it to make more of the free hard drive space
contiguous and usable for a permanent swap file.

NOTE: The maximum size limitations described in the first answer apply in
addition to the required unfragmented disk space.

NOTE: These utilities should not be run from Windows and should not be run if you
are using a disk cache. For more information, consult the manual for the utility
you are using.


KBCategory: kbusage
KBSubcategory: wfw win31 wfwg

Additional query words: 3.10 3.11 unfragment defragment swapfile

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Keywords          : win31 
Technology        : kbAudDeveloper kbWin3xSearch kbWFWSearch kbZNotKeyword3 kbWin310 kbWin311 kbWFW310 kbWFW311
Version           : :3.1,3.11

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