XP: Registry Hack to Disable or Modify Prefetch

Contributor Icon Contributed by William_Wilson Date Icon March 16, 2006  
Tag Icon Tagged: Windows

Change the amount of memory stolen by prefetch or stop it completely


Prefetch does exatly as it sounds, it loads programs, libraries and everything else the computer deems often (at one time) and loads them all into memory, whether they are commonly used now, even if they don’t exist now… these programs and files could still be using some of your memory, so let’s get it back!

This option does speed up the opening and running of popular programs, but should also be kept under control.

Clean up the prefetch folder:
navigate to C:\Windows\prefetch
-delete any heading which you do not use often enough, or if the program loads quickly without being prefetched.
*NOTE you can delete them all if you like, they will be updated again when Windows deems fit.

The items removed, will likely load a little slower than usual the first time you run them, but this is a reasonable trade off if your overall system performance is improved.

Determine how you want prefetch to run:
*This involves the registry, if you are uncomfortable with this, you should stop here.

move to path: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
On the right side look for a key named: EnablePrefetcher
This decides if and how prefetch runs, here are the value and what they do:
0: Disable
1: Prefetch when an application runs
2: Prefetch on startup or bootup
3: Always Prefetch

*Here you will have to make your own decision on what is best for your system.. i personally do a boot on app run, the length of wait is very minimal compared to any other setting, and windows startup is considerably faster! Which is what really matters.
Once a program has been added to prefetch it then will run faster the next time you open it until reboot.

Further information regarding the prefetch can be found here:
XP: Speed Up Your System - When, Where, and How to Clear the Prefetch

Questions/Comments: william_a_wilson@hotmail.com
-William. § (marvin_gohan)

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Viewing 2 Comments

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    WRONG WRONG WRONG!

    This article contains a descriprion of how prefetching works that is simple and easily understood. If only it were correct.

    Prefetch does NOT preload applications at bootup and no memory is wasted. This view of prefetching is very common but completely wrong.

    When an application starts Windows does not immediately load the entire application. Program code and data are loaded into memory only as needed and then cached for future use. This action is handled by the processor and operating system and is invisible to the application. This procedure is followed by virtually all systems that are based on virtual memory.

    This system saves considerable memory and load time, particularly with large and complex applications. But the process isn't optimum. Fragments of multiple files will be referenced in an order that is far from optimum for loading efficiently. The prefetch system was designed to solve this problem.

    When an application starts, Windows checks the prefetch folder for an associated entry. If none is found a conventional load is performed. But while this occurs Windows monitors which files are accessed and the parts that are loaded. After the application has started this data is saved in an associated prefetch file. But take note, only an index to the files is saved, not the data itself.

    When the application is run at a later time, Windows will find an associated prefetch file. The system uses the contents of this file as a guide to "prefetch" the program code and data and places it into the system cache. A normal program load is then initiated, but proceeds much faster than usual because the data is already in memory. This is what prefetching really means.

    After the prefetch folder reaches 128 application entries no more will be added. Later, during idle time, the system will examine the folder and delete all but the 32 most recently used entries. The process will then continue as before. The folder is thus self cleaning and no user action is required - or desireable.

    Prefetching this way has none of the problems that are commonly described, such as in this article. Application prefetch files are referenced only after application launch has been initiated so boot time is not prolonged. No data is loaded into memory that would not be loaded a few seconds later anyway. No memory is wasted so sytems with limited memory will not be impaired. Systems with as little as 64MB will benefit but 128 MB is requred to take full advantage of prefetching.

    The prefetch system relies on the Task Scheduler service for proper operarion so it must be enabled and set to Automatic. Some uninformed sources have said that this service is unnecessary and may be safely disabled. Doing so will cripple the prefetch system and performace degraded. Over time this degradation can become severe.

    The description of prefetch above is very much simplified and much more is actually involved. The prefetch system has been extensively tested by both Microsoft and others. My own tests confirm it. Examining the last access date of prefetch files will clearly show thay are accesed only when the program is run, not at boot time.

    Manual cleaning of prefetch files will deprive Windows of a valuable resource and serves no useful purpose. The relevant registry entries are set to optimum by default and there is nothing to be gained by changing them.

    The article by Tech-Recipes is incorrect and offers advice that will impair performance. Informed readers are well aware of this. Such serious errors must corrected.

    The article at URL: http://www.tech-recipes.com/windows_tips671.html
    is also based on the incorrect understanding of prefetch.

    References:
    http://home.comcast.net/~SupportCD/XPMyths.html
    Also check the references on this page. One is by Ryan Myers, a Microsoft engineer on the client performance team.

    I am well aware of the fact that many respected websites are promoting the same false idea as on this site. I have seen many of them. They are wrong.

    Larry Miller
    Microsoft MCSA
    CompTIA Network+, A+
    Amateur programmer.
    • ^
    • v
    Mucho thanks Larry.... I really appreciate it...
 
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