Prefetcher

The Prefetcher is a component of Microsoft Windows which was introduced in Windows XP. It is a component of the Memory Manager that can speed up the Windows boot process and shorten the amount of time it takes to start up programs. It accomplishes this by caching files that are needed by an application to RAM as the application is launched, thus consolidating disk reads and reducing disk seeks. This feature is covered by US patent 6,633,968.

Since Windows Vista, the Prefetcher has been extended by SuperFetch and ReadyBoost. SuperFetch attempts to accelerate application launch times by monitoring and adapting to application usage patterns over periods of time, and caching the majority of the files and data needed by them into memory in advance so that they can be accessed very quickly when needed. ReadyBoost (when enabled) uses external memory like a USB flash drive to extend the system cache beyond the amount of RAM installed in the computer. ReadyBoost also has a component called ReadyBoot that replaces the Prefetcher for the boot process if the system has 700 MB or more of RAM.

When a Windows system boots, components of many files need to be read into memory and processed. Often different parts of the same file (e.g. Registry hives) are loaded at different times. As a result, a significant amount of time is spent 'jumping' from file to file and back again multiple times, even though a single access would be more efficient. The prefetcher works by watching what data is accessed during the boot process (including data read from the NTFS Master File Table), and recording a trace file of this activity. The boot prefetcher will continue to watch for such activity until 30 seconds after the user's shell has started, or until 60 seconds after all services have finished initializing, or until 120 seconds after the system has booted, whichever elapses first.

Future boots can then use the information recorded in this trace file to load code and data in a more efficient fashion (i.e. by re-ordering disk reads to minimise or eliminate the need to access the same file multiple times thus minimising disk head movements).

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