How Windows Determines the Recommended Maximum Swap File Size

 Last reviewed: July 5, 1996
Article ID: Q83968
The information in this article applies to:
  • Microsoft Windows operating system versions 3.1, 3.11
  • Microsoft Windows for Workgroups versions 3.1, 3.11

SUMMARY

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

MORE INFORMATION

Temporary Swap File

1.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.
2.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

3.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 reference words: 3.10 3.11 unfragment defragment swapfile

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Last reviewed: July 5, 1996
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