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SuperFetch is a performance enhancement introduced in Microsoft Windows Vista to reduce the time necessary to launch applications. SuperFetch works with the memory manager service in Windows to analyze memory usage patterns over time to determine the optimal memory content for a given user for a date or time of day. This differs from the Prefetch technique used in Microsoft Windows XP, which preloads data into memory without analyzing usage patterns.

From Windows ReadyBoost:

SuperFetch prioritizes the following kinds of pages to remain in memory:

  • Pages of applications that are used most frequently overall.
  • Pages of applications that are commonly used when resuming:
  • After extensive hibernation (for example, first thing in the morning).
  • After shorter periods of sleep or hibernation (for example, after lunch).

Prefetched pages are added to the system’s standby page list, which has been reorganized and redesigned to retain useful data in memory over longer periods of time.

If SuperFetch detects that the system drive is a fast SSD (as measured by Windows Experience Index Disk score), then SuperFetch turns off ReadyBoot, ReadyBoost, and the SuperFetch service itself.

To calculate the Windows Experience Index Disk score run:

winsat formal


Robust performance

Robust performance (or robustness) is a component of SuperFetch to watch for specific file I/O access that might harm system performance by populating the standby lists with unneeded data.


SuperFetch distinguishes between different scenarios to accurately measure performance.

Cold scenario

In a cold scenario, the test applications are not already in memory when the test begins. Cold scenarios measure performance either after a state transition, such as boot or resume from hibernation, or after another application claims most of the available memory, such as after launching and quitting a game.

Warm scenario

In a warm scenario, some or all the scenario contents are in memory before measurement. This usually means that the test has run at least once during this logon session.

Performance scenarios

Performance scenarios defined by the Windows Performance Recorder (WPR):

  • General: Records general performance while the computer is running.
  • On/Off - Boot: Records performance while the computer is booting.
  • On/Off – Fast Startup: Records performance during a fast startup.
  • On/Off - Shutdown: Records performance while shutting the computer down.
  • On/Off - RebootCycle: Records performance during the entire cycle while the computer is rebooting.
  • On/Off - Standby/Resume: Records performance when the computer is placed on standby and then resumed.
  • On/Off - Hibernate/Resume: Records performance when the computer is placed in hibernation and then resumed.

Where "On/Off" likely refers to a cold scenario.


Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the Registry value:

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
Value: EnableSuperfetch

A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, services.msc 1.

File Formats

Data for SuperFetch is gathered by the %SystemRoot%\System32\Sysmain.dll, part of the Service Host process, %SystemRoot%\System32\Svchost.exe, and stored in a series of files in the %SystemRoot%\Prefetch directory 2. These files appear to start with the prefix Ag and have a .db extension. Note that there are likely more SuperFetch database files named differently, presumably all using the .db extension.

The format of the SuperFetch database files is not fully known, there is available unofficial partial specification 3 and open source (GPL) dumper for .db files 4. For more information see SuperFetch Format.

The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed 5.

The SuperFetch service is managed by the File Information FS MiniFilter service. It appears that most of the SuperFetch database files are updated (written) when the service is shut down. AgAppLaunch.db is also written when the service starts.

See Also


Open Source