Ensuring That You Have an Adequate Virtual Memory Configuration

Physical memory might be the vital lubricant of a happily humming Windows machine, but Windows is not designed to run on RAM chips alone, no matter how many of them you have. In addition to using physical RAM to store programs and data, Windows creates a hidden file on your primary hard disk and uses that file to manage pages of data pulled from scattered sections of the hard disk and used in physical memory when necessary. The page file acts as an extension of main memory—or, in other words, as virtual memory.

In olden days (especially in the early to mid 1990s), the memory manager's disk-backed storage was commonly called the "swap file," because its primary use was to overcome physical memory shortages Today, the page file is an integral part of memory management, used by SuperFetch and the boot prefetcher to optimize your system for performance

In Windows 7, Microsoft has chosen to deemphasize visible measurements of page file usage in common performance-monitoring tools. The Commit fraction in the lower right corner of Windows Task Manager's Performance tab is useful for helping you gauge the adequacy of your virtual memory setup . Note, however, that while the numerator of the fraction indicates how much virtual memory your system is currently using, the denominator reports the sum of physical memory and current page-file size.

In a default installation, Windows creates the page file in the root folder on the same drive that holds the Windows system files . The size of the page file is determined by the amount of RAM in your system . By default, the minimum size on a 32-bit (x86) system is 1 .5 times the amount of physical RAM if physical RAM is less than 1 GB, and equal to the amount of physical RAM plus 300 MB if 1 GB or more is installed. The default maximum size is three times the amount of RAM, regardless of how much physical RAM is installed. On a PC with a processor that supports Physical Address Extension (PAE)—which is to say, on any PC that is capable of running Windows 7—the maximum size of the page file is 16 TB . That amount of disk space will no doubt seem horribly confining someday, perhaps even in our lifetimes, but for now it's more than enough . You can see the page file in a Windows Explorer window if you configure Windows to show hidden and system files; look for Pagefile. sys in the root of your system drive.

To see the current configuration of your system's virtual memory, open the System dialog box in Control Panel and click the Advanced tab . (For an excellent, undocumented shortcut to this dialog box, click Start, type systempropertiesadvanced with no spaces, and press Enter.) Under the Performance heading, click Settings . In the Performance Options dialog box, click the Advanced tab and (finally!) under the Virtual Memory heading, click Change . Figure 20-7 shows the Virtual Memory dialog box, with default settings for a machine with 2 GB of RAM (default, that is, except that we cleared the Automatically Manage Paging File Size For All Drives check box to make the rest of the dialog box easier to read).

By default, Windows creates a single page file in the root folder on the same volume that holds the Windows system files and manages its size for you. The Currently Allocated number near the bottom of the dialog box shows you how large the file is now. If conditions on your system change (you run an unusually large assortment of memory-intensive applications, for example), Windows might expand the page file . It might then return the file to its original size (or a smaller size) if the demand subsides. All this happens without intervention or notification if you leave the Automatically Manage Paging File Size For All Drives check box selected

Virtual Memory

Automatically manage paging file size for all drives Paging file size for each drive Drive [Volume Label] Paging File Size (ME)

r:

System managed

D:

[TV]

None

n

F:

None

G:

None

H:

None

I:

[SD CARD]

None

Space available: 112245MB

Selected drive: C:

Space available: 112245MB

Maximum size (MB): [j (Ö System managed size No paging file

Total paging file size for all drives Minimum allowed: 16 MB Recommended: 3067 MB Currently allocated: 2045 MB

OK I I Cancel

OK I I Cancel

Figure 20-7 By default, Windows manages the page file size for you; clear the check box at the top of this dialog box to manage the page file manually.

If you don't want Windows to do this for you, you have the following options:

• You can move the page file to a different volume if you have more than one.

• If you have more than one volume, you can establish more than one page file.

• For any page file, you can choose between System Managed Size and Custom Size.

• If you choose Custom Size, you can specify an initial size and a maximum size .

• You can remove a paging file from a volume by selecting the volume and choosing No Paging File. (You can even get rid of all paging files this way, although doing so is not recommended, even on systems with a lot of RAM.)

Should you get involved in page-file management, and, if so, how?

If you have more than one physical disk, moving the page file to a fast drive that doesn't contain your Windows system files is a good idea. Using multiple page files split over two or more physical disks is an even better idea, because your disk controller can process multiple requests to read or write data concurrently. Don't make the mistake of creating two or more page files using multiple volumes on a single physical disk, however. If you have a single hard disk that contains C, D, and E volumes, for example, and you split the page file over two or more of these, you might actually make your computer run more slowly than before . In that configuration, the heads on the physical disk have to do more work, loading pages from different portions of the same disk sequentially, rather than loading data from a single contiguous region of the hard disk.

If you are short of hard disk space, you might consider setting a smaller initial page file size Monitor peak usage levels over time; if the peak is well below the current page file size, you can consider reducing the initial size to save disk space. On the other hand, if you're not short of disk space, there's nothing to be gained from doing this and you might occasionally overload your custom settings, thereby degrading the performance of your system .

Should you enlarge your page file? Most users won't need to do this . But you might want to keep an eye on the green line in the Memory chart on the Overview tab of Resource ic

Monitor (see the full discussion under "Using Resource Monitor" on page 716). If that line is spiking off the top of the graph a great deal of the time during your normal work, you might consider increasing the maximum size of your page file. (Disregard page file spikes and disk activity in general that takes place while you're not actually working. This is likely to be the result of search indexing, defragmentation, or other background processes and does not indicate a problem with your actual work performance.)

Note

For an extremely detailed discussion of virtual memory page file management in Windows Vista, we recommend the blog post "Pushing the Limits of Windows: Virtual Memory?" by Microsoft Technical Fellow Mark Russinovich, at w7io.com/2004.

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