When you are in the process of implementing Windows Vista in an enterprise environment, you may run into the following situation:
You have a logon script that writes a file to disk or queries AD with ADSI. For no apparent reason the script hangs for minutes before it finishes without any error message. On further investigation you will find no issues regarding the network, memory or disk resources. When you manually start the script it always runs fast without issues. What is happening here?
Get ready for a long story.
Windows Vista has a number features that are supposed to improve the user experience during system startup and logon. One of these is Boxing. Boxing is a feature that makes sure that programs that start while you logon are no longer able to start fighting for all available resources eating all disk resources and CPU. Usually programs like the Adobe Updater, Winzip Quicklauncher, iTunes quicklaunch or whatever these programs are called, are not really of interest for the user in terms of how fast they startup. But they will slow down the system significantly when Windows threats them just as equal as you manually try to start after logon (like Outlook or Word for example). Windows Vista lowers the thread priority level for startup applications to " Below Normal" and will not allow them to raise their thread priority during the first 60 seconds after logon. In this way Vista slows down the execution of these applications and leaves resources available for programs that the user actually wants to start.
So far so good. Vista uses this mechanism for programs started from the following locations:
- Startup folder in the Start Menu
- Windows\CurrentVersion\Run key in the Registry
- Scheduled tasks initiated at Logon
- Defined in Group Policy in: User Configuration | Administrative Templates | System| Logon | Run these programs a user logon
Windows Vista actually lowers the thread priority level of these applications to BelowNormal and disables their normal ability to bump up their own thread level during the first minute after logon. Maarten from the Vista product team wrote a little piece about this behavior in their blog. At the same time Vista lowers the application' s I/O priority level to "very low I/O priority level". This is a new feature that did not exist before Windows Vista. Normally a process that writes a file to disk does not directly access the disk, but uses a mechanism called the file cache. When using the file cache, a program writes data into a RAM based memory cache first. Then Windows will take care of actually writing the data to disk when it's convenient for the system. Writing data to the file cache is really fast and programs usually hardly wait for data being written to the file cache. Processes running at the very low I/O priority level do not make use of the file cache, but write their data directly to disk when the Windows file system driver thinks it is convenient. This makes writing data to disk very very slow while the application is being "boxed".