Artigo Mitos sobre o Prefetch

One more time: do not clean out your Prefetch folder!

Yet another Web site posted yet another “tip” today recommending that you clean out your Prefetch folder to improve performance of Windows. Arrrggghhh! I’ve written about this repeatedly (here and here and here, for instance), but the message doesn’t seem to be spreading very fast. Maybe this quote from “Misinformation and the Prefetch Flag” by Ryan Myers, a developer on Microsoft’s Windows Client Performance Team, will help:

|XP systems have a Prefetch directory underneath the windows root directory, full of .pf files — these are lists of pages to load. The file names are generated from hashing the EXE to load — whenever you load the EXE, we hash, see if there’s a matching (exename)-(
hash).pf file in the prefetch directory, and if so we load those pages. (If it doesn’t exist, we track what pages it loads, create that file, and pick a handful of them to save to it.) So, first off, it is a bad idea to
periodically clean out that folder as some tech sites suggest. For one thing, XP will just re-create that data anyways; secondly, it trims the files anyways if there’s ever more than 128 of them so that it doesn’t needlessly consume space. So not only is deleting the directory totally unnecessary, but you’re also putting a temporary dent in your PC’s performance. [emphasis in original]

Bottom line: You will not improve Windows performance by cleaning out the Prefetch folder. You will, in fact, degrade Windows performance by cleaning out the Prefetch folder. I’ve done performance testing that establishes this definitively. In all the many sites that offer this bogus tip, I have yet to see a single piece of actual performance testing.
Oh, and for anyone who cites this TechRepublic article as a source, let me just say that it contains more serious factual errors than I can count. For instance:

|As you boot your workstation or access programs on your workstation, XP’s prefetcher copies portions of those files to the Prefetch area of your hard drive.

That’s completely wrong. The files in the Prefetch folder contain lists of pages that that should be loaded when a program starts. Each file is essentially an index. Windows XP doesn’t copy portions of any files to the Prefetch folder.

|When your workstation boots, XP prefetches portions of the files you use most frequently and has any application you’ve recently run waiting and ready to go.

This is equally absurd. If this were true, it would mean that Windows was actually loading into memory every program you’ve ever used, every time you start Windows. That’s not the way it works at all. When your PC starts up, Windows looks in the Prefetch folder to determine how best to load Windows. It doesn’t do a thing with the .pf files for applications (unless, of course, you’ve configured one of those apps to start up with Windows).

|If you’re frequently using the same few applications over and over again, prefetching can greatly increase the apparent speed of a system. Rather than waiting for you to click an icon to start a program, and then loading all of the associated files, libraries, and pointers necessary to run the program, XP has all the components of your programs preloaded. When you click an icon to start the program, most of the hard work is already done.

The author just made this up. The .pf files don’t get used at all until you run a program. What actually happens when you click an icon is that Windows uses the information in the Prefetch folder to decide which program segments to load and in what order to load those pages. There’s plenty of documentation for this, including Ryan Myers’ article and this definitive article by Mark Russinovitch and David Solomon, Windows XP Kernel Improvements Create a More Robust, Powerful, and Scalable OS.

|The drawback to prefetching is that XP will prefetch a program even if you use it only once or twice. XP will retain a copy of a portion of it in the Prefetch folder. From there, it will prefetch the program, taking resources from your workstation even though you may have no intention of ever using the program again.

Again, the author just pulled this out of who-knows-where. When you run a program, Windows creates a .pf file for it in the Prefetch folder. When you run the program again, Windows looks for this .pf file and uses it to determine how to load the program. The hash doesn’t contain any portion of the original program code. If you never run the program again, that .pf file never gets used, and in fact it gets deleted eventually.
I used to write for TechRepublic. I’ve tried to contact someone there to get them to correct this silly article but have yet to receive a response. It would be really, really great if some of the other sites that have propagated this urban legend would also correct it.

Como refere o autor, trata-se de um erro factual muito comum e este artigo não expressa uma opinião, mas sim factos reais, sendo realmente deste modo que o Windows actua, como muitos aqui provavelmente saberão.

Além disso, encontram-se aqui na Techzone guias sobre como aumentar a velocidade das aplicações (como jogos), nos quais se dá exactamente este conselho, erradamente, pois apesar de poder eventualmente reduzir um pouco o tempo de arranque, não o faz em relação às mesmas, fazendo simplesmente o contrário. Como tal, fica a sugestão de Sticky Thread. ;)

No artigo original, podem ser encontrados os comentários sobre o tema.
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Power Member
Bom post.. Eu normalmente na loja (estagio) costumo apagar o conteudo dessa pasta.. É um dos procedimentos (isto dito pelo tecnico).. Bem, mas toda a gente pode errar,certo?
Pode-se apagar quando se pretende fazer um reset ao Prefetch da máquina... mas em 90% dos casos, tal não vale a pena!

Sim, é verdade, o mito erróneo centra-se na crença de que as bibliotecas e mesmo executáveis são previamente abertos e carregados na memória, o que não é verdade, ocorrendo apenas uma optimização quando, e só quando, a aplicação é aberta pelo utilizador.