Windows Administration: Inside the Windows Vista Kernel: Part 2

Windows Administration: Inside the Windows Vista Kernel: Part 2:

'via Blog this'

SuperFetch

"A significant change to the Memory Manager is in the way that it manages physical memory. The Standby List management used by previous versions of Windows has two limitations. First, the prioritization of pages relies only on the recent past behavior of processes and does not anticipate their future memory requirements. Second, the data used for prioritization is limited to the list of pages owned by a process at any given point in time. These shortcomings can result in scenarios like the "after lunch syndrome," where you leave your computer for a while and a memory-intensive system application runs (such as an antivirus scan or disk defragmentation). This application forces the code and data that your active applications had cached in memory to be overwritten by the memory-intensive activities. When you return, you experience sluggish performance as applications have to request their data and code from disk.

Windows XP introduced prefetching support that improved boot and application startup performance by performing large disk I/Os to preload memory with code and file system data that it expected, based on previous boots and application launches. Windows Vista goes a big step further with SuperFetch, a memory management scheme that enhances the least-recently accessed approach with historical information and proactive memory management.

SuperFetch is implemented in %SystemRoot%\System32\Sysmain.dll as a Windows service that runs inside a Service Host process (%SystemRoot%\System32\Svchost.exe). The scheme relies on support from the Memory Manager so that it can retrieve page usage histories as well as direct the Memory Manager to preload data and code from files on disk or from a paging file into the Standby List and assign priorities to pages. The SuperFetch service essentially extends page-tracking to data and code that was once in memory, but that the Memory Manager has reused to make room for new data and code. It stores this information in scenario files with a .db extension in the %SystemRoot%\Prefetch directory alongside standard prefetch files used to optimize application launch. Using this deep knowledge of memory usage, SuperFetch can preload data and code when physical memory becomes available.

Whenever memory becomes free-for example, when an application exits or releases memory-SuperFetch asks the Memory Manager to fetch data and code that was recently evicted. This is done at a rate of a few pages per second with Very Low priority I/Os so that the preloading does not impact the user or other active applications. Therefore, if you leave your computer to go to lunch and a memory-intensive background task causes the code and data from your active applications to be evicted from memory while you're gone, SuperFetch can often bring all or most of it back into memory before you return. SuperFetch also includes specific scenario support for hibernation, standby, Fast User Switching (FUS), and application launch. When the system hibernates, for example, SuperFetch stores data and code in the hibernation file that it expects (based on previous hibernations) will be accessed during the subsequent resume. In contrast, when you resume Windows XP, previously cached data must be reread from the disk when it is referenced."