Windows® Internals, Sixth Edition, Part 2

However, a demand-paging policy can result in a process incurring many page faults when its threads first begin executing or when they resume execution at a later point. To optimize the startup of a process (and the system), Windows has an intelligent prefetch engine called the logical prefetcher, described in the next section. Further optimization and prefetching is performed by another component, called Superfetch, that we’ll describe later in the chapter.

Logical Prefetcher

During a typical system boot or application startup, the order of faults is such that some pages are brought in from one part of a file, then perhaps from a distant part of the same file, then from a different file, perhaps from a directory, and then again from the first file. This jumping around slows down each access considerably and, thus, analysis shows that disk seek times are a dominant factor in slowing boot and application startup times. By prefetching batches of pages all at once, a more sensible ordering of access, without excessive backtracking, can be achieved, thus improving the overall time for system and application startup. The pages that are needed can be known in advance because of the high correlation in accesses across boots or application starts.

The prefetcher tries to speed the boot process and application startup by monitoring the data and code accessed by boot and application startups and using that information at the beginning of a subsequent boot or application startup to read in the code and data. When the prefetcher is active, the memory manager notifies the prefetcher code in the kernel of page faults, both those that require that data be read from disk (hard faults) and those that simply require data already in memory be added to a process’s working set (soft faults). The prefetcher monitors the first 10 seconds of application startup. For boot, the prefetcher by default traces from system start through the 30 seconds following the start of the user’s shell (typically Explorer) or, failing that, up through 60 seconds following Windows service initialization or through 120 seconds, whichever comes first.

The trace assembled in the kernel notes faults taken on the NTFS master file table (MFT) metadata file (if the application accesses files or directories on NTFS volumes), on referenced files, and on referenced directories. With the trace assembled, the kernel prefetcher code waits for requests from the prefetcher component of the Superfetch service (%SystemRoot%\System32\Sysmain.dll), running in a copy of Svchost. The Superfetch service is responsible for both the logical prefetching component in the kernel and for the Superfetch component that we’ll talk about later. The prefetcher signals the event \KernelObjects\PrefetchTracesReady to inform the Superfetch service that it can now query trace data.

Note

You can enable or disable prefetching of the boot or application startups by editing the DWORD registry value HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters\EnablePrefetcher. Set it to 0 to disable prefetching altogether, 1 to enable prefetching of only applications, 2 for prefetching of boot only, and 3 for both boot and applications.

The Superfetch service (which hosts the logical prefetcher, although it is a completely separate component from the actual Superfetch functionality) performs a call to the internal NtQuerySystemInformation system call requesting the trace data. The logical prefetcher post-processes the trace data, combining it with previously collected data, and writes it to a file in the %SystemRoot%\Prefetch folder, which is shown in Figure 10-48. The file’s name is the name of the application to which the trace applies followed by a dash and the hexadecimal representation of a hash of the file’s path. The file has a .pf extension; an example would be NOTEPAD.EXE-AF43252301.PF.

There are two exceptions to the file name rule. The first is for images that host other components, including the Microsoft Management Console (%SystemRoot%\System32\Mmc.exe), the Service Hosting Process (%SystemRoot%\System32\Svchost.exe), the Run DLL Component (%SystemRoot%\System32\Rundll32.exe), and Dllhost (%SystemRoot%\System32\Dllhost.exe). Because add-on components are specified on the command line for these applications, the prefetcher includes the command line in the generated hash. Thus, invocations of these applications with different components on the command line will result in different traces.

The other exception to the file name rule is the file that stores the boot’s trace, which is always named NTOSBOOT-B00DFAAD.PF. (If read as a word, “boodfaad” sounds similar to the English words boot fast.) Only after the prefetcher has finished the boot trace (the time of which was defined earlier) does it collect page fault information for specific applications.