Difference between pages "Windows Prefetch File Format" and "SuperFetch"

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(Metrics entry record - version 17)
 
(Components)
 
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{{expand}}
+
{{Expand}}
  
A Windows Prefetch file consists of one file header and multiple file sections with different content. Not all content has an obvious forensic value.
+
SuperFetch is a performance enhancement introduced in [[Microsoft]] [[Windows|Windows Vista]] to reduce the time necessary to launch applications. SuperFetch works with the memory manager service in Windows to analyze memory usage patterns over time to determine the optimal memory content for a given user for a date or time of day. This differs from the [[Prefetch]] technique used in Microsoft Windows XP, which preloads data into memory without analyzing usage patterns.
  
As far as have been possible to ascertain, there is no public description of the format. The description below has been synthesised from examination
+
From [http://msdn.microsoft.com/en-us/library/windows/hardware/dn653317(v=vs.85).aspx]: SuperFetch prioritizes the following kinds of pages to remain in memory:
of multiple prefetch files.
+
* Pages of applications that are used most frequently overall.
 +
* Pages of applications that are commonly used when resuming:
 +
** After extensive hibernation (for example, first thing in the morning).
 +
** After shorter periods of sleep or hibernation (for example, after lunch).  
  
== Characteristics ==
+
If SuperFetch detects that the system drive is a fast SSD (as measured by Windows Experience Index Disk score), then SuperFetch turns off [[ReadyBoot]], [[ReadyBoost]], and the SuperFetch service itself.
{| class="wikitable"
+
|-
+
| <b>Integers</b>
+
| stored in little-endian
+
|-
+
| <b>Strings</b>
+
| Stored as [http://en.wikipedia.org/wiki/UTF-16/UCS-2 UTF-16 little-endian] without a byte-order-mark (BOM).
+
|-
+
| <b>Timestamps</b>
+
| Stored as [http://msdn2.microsoft.com/en-us/library/ms724284.aspx Windows FILETIME] in UTC.
+
|-
+
|}
+
  
== File header ==
+
== Components ==
The file header is 84 bytes of size and consists of:
+
=== Robust performance ===
{| class="wikitable"
+
Robust performance (or robustness) is a component of SuperFetch to watch for specific file I/O access that might harm system performance by populating the standby lists with unneeded data.
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
| H1
+
| 0x0000
+
| 4
+
| DWORD
+
| Format version (see format version section below)
+
|-
+
| H2
+
| 0x0004
+
| 4
+
| DWORD
+
| Signature 'SCCA' (or in hexadecimal representation 0x53 0x43 0x43 0x4)
+
|-
+
| H3
+
| 0x0008
+
| 4
+
| DWORD?
+
| Unknown - Values observed: 0x0F - Windows XP, 0x11 - Windows 7, Windows 8.1
+
|-
+
| H4
+
| 0x000C
+
| 4
+
| DWORD
+
| Prefetch file size (or length) (sometimes referred to as End of File (EOF)).
+
|-
+
| H5
+
|0x0010
+
| 60
+
| USTR
+
| The name of the (original) executable as a Unicode (UTF-16 litte-endian string), up to 29 characters and terminated by an end-of-string character (U+0000). This name should correspond with the one in the prefetch file filename.
+
|-
+
| H6
+
|0x004C
+
|4
+
|DWORD
+
|The prefetch hash. This hash value should correspond with the one in the prefetch file filename.
+
|-
+
| H7
+
|0x0050
+
|4
+
|?
+
| Unknown (flags)? Values observed: 0 for almost all prefetch files (XP); 1 for NTOSBOOT-B00DFAAD.pf (XP)
+
|-
+
|}
+
  
It's worth noting that the name of a carved prefetch file can be restored using the information in field H5 and H6, and its size can be determined by field H4.
+
== Scenarios ==
 +
SuperFetch distinguishes between different scenarios to accurately measure performance.
  
=== Format version ===
+
=== Cold scenario ===
 +
In a cold scenario, the test applications are not already in memory when the test begins. Cold scenarios measure performance either after a state transition, such as boot or resume from hibernation, or after another application claims most of the available memory, such as after launching and quitting a game.
  
{| class="wikitable"
+
=== Warm scenario ===
|-
+
In a warm scenario, some or all the scenario contents are in memory before measurement. This usually means that the test has run at least once during this logon session.
! Value
+
! Windows version
+
|-
+
| 17 (0x11)
+
| Windows XP, Windows 2003
+
|-
+
| 23 (0x17)
+
| Windows Vista, Windows 7
+
|-
+
| 26 (0x1a)
+
| Windows 8.1 (note this could be Windows 8 as well but has not been confirmed)
+
|-
+
|}
+
  
=== File information ===
+
== Configuration ==
The format of the file information is version dependent.
+
  
Note that some other format specifications consider the file information part of the file header.  
+
Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the [[Registry]] value [http://www.codinghorror.com/blog/archives/000688.html]:
 +
<pre>
 +
Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
 +
Value: EnableSuperfetch
 +
</pre>
  
==== File information - version 17 ====
+
A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, <tt>services.msc</tt> [http://tiredblogger.wordpress.com/2007/03/27/superfetch-not-so-super-for-gaming/].
The file information – version 17 is 68 bytes of size and consists of:
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0x0054
+
| 4
+
| DWORD
+
| The offset to section A. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0058
+
| 4
+
| DWORD
+
| The number of entries in section A.
+
|-
+
|
+
| 0x005C
+
| 4
+
| DWORD
+
| The offset to section B. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0060
+
| 4
+
| DWORD
+
| The number of entries in section B.
+
|-
+
|
+
| 0x0064
+
| 4
+
| DWORD
+
| The offset to section C. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0068
+
| 4
+
| DWORD
+
| Length of section C.
+
|-
+
|
+
| 0x006C
+
| 4
+
| DWORD
+
| Offset to section D. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0070
+
| 4
+
| DWORD
+
| The number of entries in section D.
+
|-
+
|
+
| 0x0074
+
| 4
+
| DWORD
+
| Length of section D.
+
|-
+
|
+
| 0x0078
+
| 8
+
| FILETIME
+
| Latest execution time (or run time) of executable (FILETIME)
+
|-
+
|
+
| 0x0080
+
| 16
+
| ?
+
| Unknown ? Possibly structured as 4 DWORD. Observed values: /0x00000000 0x00000000 0x00000000 0x00000000/, /0x47868c00 0x00000000 0x47860c00 0x00000000/ (don't exclude the possibility here that this is remnant data)
+
|-
+
|
+
| 0x0090
+
| 4
+
| DWORD
+
| Execution counter (or run count)
+
|-
+
|
+
| 0x0094
+
| 4
+
| DWORD?
+
| Unknown ? Observed values: 1, 2, 3, 4, 5, 6 (XP)
+
|-
+
|}
+
  
==== File information - version 23 ====
+
== File Formats ==
The file information – version 23 is 156 bytes of size and consists of:
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0x0054
+
| 4
+
| DWORD
+
| The offset to section A. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0058
+
| 4
+
| DWORD
+
| The number of entries in section A.
+
|-
+
|
+
| 0x005C
+
| 4
+
| DWORD
+
| The offset to section B. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0060
+
| 4
+
| DWORD
+
| The number of entries in section B.
+
|-
+
|
+
| 0x0064
+
| 4
+
| DWORD
+
| The offset to section C. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0068
+
| 4
+
| DWORD
+
| Length of section C.
+
|-
+
|
+
| 0x006C
+
| 4
+
| DWORD
+
| Offset to section D. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0070
+
| 4
+
| DWORD
+
| The number of entries in section D.
+
|-
+
|
+
| 0x0074
+
| 4
+
| DWORD
+
| Length of section D.
+
|-
+
|
+
| <b>0x0078</b>
+
| <b>8</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|
+
| 0x0080
+
| 8
+
| FILETIME
+
| Latest execution time (or run time) of executable (FILETIME)
+
|-
+
|
+
| 0x0088
+
| 16
+
| ?
+
| Unknown
+
|-
+
|
+
| 0x0098
+
| 4
+
| DWORD
+
| Execution counter (or run count)
+
|-
+
|
+
| 0x009C
+
| 4
+
| DWORD?
+
| Unknown
+
|-
+
|
+
| <b>0x00A0</b>
+
| <b>80</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|}
+
  
==== File information - version 26 ====
+
Data for SuperFetch is gathered by the <tt>%SystemRoot%\System32\Sysmain.dll</tt>, part of the Service Host process, <tt>%SystemRoot%\System32\Svchost.exe</tt>, and stored in a series of files in the <tt>%SystemRoot%\Prefetch</tt> directory [http://www.microsoft.com/technet/technetmag/issues/2007/03/VistaKernel/]. These files appear to start with the prefix <tt>Ag</tt> and have a <tt>.db</tt> extension. Note that there are likely more SuperFetch database files named differently, presumably all using the .db extension.
The file information – version 26 is 224 bytes of size and consists of:
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0x0054
+
| 4
+
| DWORD
+
| The offset to section A. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0058
+
| 4
+
| DWORD
+
| The number of entries in section A.
+
|-
+
|
+
| 0x005C
+
| 4
+
| DWORD
+
| The offset to section B. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0060
+
| 4
+
| DWORD
+
| The number of entries in section B.
+
|-
+
|
+
| 0x0064
+
| 4
+
| DWORD
+
| The offset to section C. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0068
+
| 4
+
| DWORD
+
| Length of section C.
+
|-
+
|
+
| 0x006C
+
| 4
+
| DWORD
+
| Offset to section D. The offset is relative from the start of the file.
+
|-
+
|
+
| 0x0070
+
| 4
+
| DWORD
+
| The number of entries in section D.
+
|-
+
|
+
| 0x0074
+
| 4
+
| DWORD
+
| Length of section D.
+
|-
+
|
+
| 0x0078
+
| 8
+
| ?
+
| Unknown
+
|-
+
|
+
| 0x0080
+
| 8
+
| FILETIME
+
| Latest execution time (or run time) of executable (FILETIME)
+
|-
+
|
+
| <b>0x0088</b>
+
| <b>7 x 8 = 56</b>
+
| <b>FILETIME</b>
+
| <b>Older (most recent) latest execution time (or run time) of executable (FILETIME)</b>
+
|-
+
|
+
| <b>0x00C0</b>
+
| <b>16</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|
+
| 0x00D0
+
| 4
+
| DWORD
+
| Execution counter (or run count)
+
|-
+
|
+
| <b>0x00D4</b>
+
| <b>4</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|
+
| <b>0x00D8</b>
+
| <b>4</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|
+
| <b>0x00DC</b>
+
| <b>88</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|}
+
  
== Section A - Metrics array ==
+
The format of the SuperFetch database files is not fully known, there is available unofficial partial specification [http://blog.rewolf.pl/blog/?p=214] and open source (GPL) dumper for .db files [http://code.google.com/p/rewolf-superfetch-dumper/]. For more information see [[Windows SuperFetch Format|SuperFetch Format]].
==== Metrics entry record - version 17 ====
+
The metrics entry records – version 17 is 20 bytes in size and consists of:
+
  
{| class="wikitable"
+
The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed [http://channel9.msdn.com/showpost.aspx?postid=242429].
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0
+
| 4
+
| DWORD
+
| Start time in ms
+
|-
+
|
+
| 4
+
| 4
+
| DWORD
+
| Duration in ms
+
|-
+
|
+
| 8
+
| 4
+
| DWORD
+
| Average duration in ms?
+
|-
+
|
+
| 12
+
| 4
+
| DWORD
+
| Filename string offset <br> The offset is relative to the start of the filename string section (section C)
+
|-
+
|
+
| 16
+
| 4
+
| DWORD
+
| Filename string number of characters without end-of-string character
+
|}
+
  
==== Metrics entry record - version 23 ====
+
The SuperFetch service is managed by the File Information FS MiniFilter service. It appears that most of the SuperFetch database files are updated (written) when the service is shut down. AgAppLaunch.db is also written when the service starts.
The metrics entry records – version 23 is 32 bytes in size and consists of:
+
 
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0
+
| 4
+
| DWORD
+
| Start time in ms
+
|-
+
|
+
| 4
+
| 4
+
| DWORD
+
| Duration in ms
+
|-
+
|
+
| 8
+
| 4
+
| DWORD
+
| Average duration in ms?
+
|-
+
|
+
| 12
+
| 4
+
| DWORD
+
| Filename string offset <br> The offset is relative to the start of the filename string section (section C)
+
|-
+
|
+
| 16
+
| 4
+
| DWORD
+
| Filename string number of characters without end-of-string character
+
|-
+
|
+
| 20
+
| 4
+
| DWORD
+
| Unknown, flags?
+
|-
+
|
+
| 24
+
| 8
+
|
+
| NTFS file reference <br> 0 if not set.
+
|}
+
 
+
==== Metrics entry record - version 26 ====
+
The metrics entry record – version 26 appears to be similar to metrics entry record – version 23.
+
 
+
== Section B - Trace chains array ==
+
This section contains an array with 12 byte (version 17, 23 and 26) entry records.
+
 
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0
+
| 4
+
|
+
| Next array entry index <br> Contains the next trace chain array entry index in the chain, where the first entry index starts with 0, or -1 (0xffffffff) for the end-of-chain.
+
|-
+
|
+
| 4
+
| 4
+
|
+
| Total block load count <br> Number of blocks loaded (or fetched) <br> The block size 512k (512 x 1024) bytes
+
|-
+
|
+
| 8
+
| 1
+
|
+
| Unknown
+
|-
+
|
+
| 9
+
| 1
+
|
+
| Sample duration in ms?
+
|-
+
|
+
| 10
+
| 2
+
|
+
| Unknown
+
|}
+
 
+
== Section C - Filename strings ==
+
This section contains filenames strings, it consists of an array of UTF-16 little-endian formatted strings with end-of-string characters (U+0000).
+
 
+
At the end of the section there seems to be alignment padding that can contain remnant values.
+
 
+
== Section D - Volumes information (block) ==
+
 
+
Section D contains one or more subsections, each subsection refers to directories on a volume.
+
 
+
If all the executables and libraries referenced in the C section are from one single disk volume, there will be only one section in the D section. If multiple volumes are referenced by section C, section D will contain multiple sections.  (A simple way to force this situation is to copy, say, NOTEPAD.EXE to a USB drive, and start it from that volume. The corresponding prefetch file will have one D header referring to, e.g. \DEVICE\HARDDISK1\DP(1)0-0+4 (the USB drive), and one to, e.g. \DEVICE\HARDDISKVOLUME1\ (where the .DLLs and other support files were found).
+
 
+
In this section, all offsets are assumed to be counted from the start of the D section.
+
 
+
=== Volume information ===
+
The structure of the volume information is version dependent.
+
 
+
==== Volume information - version 17 ====
+
The volume information – version 17 is 40 bytes in size and consists of:
+
 
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
| VI1
+
| +0x0000
+
| 4
+
| DWORD
+
| Offset to volume device path (Unicode, terminated by U+0000)
+
|-
+
| VI2
+
| +0x0004
+
| 4
+
| DWORD
+
| Length of volume device path (nr of characters, including terminating U+0000)
+
|-
+
| VI3
+
| +0x0008
+
| 8
+
| FILETIME
+
| Volume creation time.
+
|-
+
| VI4
+
| +0x0010
+
| 4
+
| DWORD
+
| Volume serial number of volume indicated by volume string
+
|-
+
| VI5
+
| +0x0014
+
| 4
+
| DWORD
+
| Offset to sub section E
+
|-
+
| VI6
+
| +0x0018
+
| 4
+
| DWORD
+
| Length of sub section E (in bytes)
+
|-
+
| VI7
+
| +0x001C
+
| 4
+
| DWORD
+
| Offset to sub section F
+
|-
+
| VI8
+
| +0x0020
+
| 4
+
| DWORD
+
| Number of strings in sub section F
+
|-
+
| VI9
+
| +0x0024
+
| 4
+
| ?
+
| Unknown
+
|-
+
|}
+
 
+
==== Volume information - version 23 ====
+
The volume information entry – version 23 is 104 bytes in size and consists of:
+
 
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
| VI1
+
| +0x0000
+
| 4
+
| DWORD
+
| Offset to volume device path (Unicode, terminated by U+0000)
+
|-
+
| VI2
+
| +0x0004
+
| 4
+
| DWORD
+
| Length of volume device path (nr of characters, including terminating U+0000)
+
|-
+
| VI3
+
| +0x0008
+
| 8
+
| FILETIME
+
| Volume creation time.
+
|-
+
| VI4
+
| +0x0010
+
| 4
+
| DWORD
+
| Volume serial number of volume indicated by volume string
+
|-
+
| VI5
+
| +0x0014
+
| 4
+
| DWORD
+
| Offset to sub section E
+
|-
+
| VI6
+
| +0x0018
+
| 4
+
| DWORD
+
| Length of sub section E (in bytes)
+
|-
+
| VI7
+
| +0x001C
+
| 4
+
| DWORD
+
| Offset to sub section F
+
|-
+
| VI8
+
| +0x0020
+
| 4
+
| DWORD
+
| Number of strings in sub section F
+
|-
+
| VI9
+
| +0x0024
+
| 4
+
| ?
+
| Unknown
+
|-
+
| <b>VI10</b>
+
| <b>+0x0028</b>
+
| <b>28</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
| <b>VI11</b>
+
| <b>+0x0044</b>
+
| <b>4</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
| <b>VI12</b>
+
| <b>+0x0048</b>
+
| <b>28</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
| <b>VI13</b>
+
| <b>+0x0064</b>
+
| <b>4</b>
+
| <b>?</b>
+
| <b>Unknown</b>
+
|-
+
|}
+
 
+
==== Volume information - version 26 ====
+
The volume information entry – version 26 appears to be similar to volume information – version 23.
+
 
+
=== Sub section E - NTFS file references ===
+
This sub section can contain NTFS file references.
+
 
+
For more information see [https://googledrive.com/host/0B3fBvzttpiiSbl9XZGZzQ05hZkU/Windows%20Prefetch%20File%20(PF)%20format.pdf Windows Prefetch File (PF) format].
+
 
+
=== Sub section F - Directory strings ===
+
This sub sections contains directory strings. The number of strings is stored in the volume information.
+
 
+
A directory string is stored in the following structure:
+
{| class="wikitable"
+
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
|
+
| 0x0000
+
| 2
+
| DWORD
+
| Number of characters (WORDs) of the directory name. The value does not include the end-of-string character.
+
|-
+
|
+
| 0x0002
+
|
+
| USTR
+
| The directory name as a Unicode (UTF-16 litte-endian string) terminated by an end-of-string character (U+0000).
+
|-
+
|}
+
  
 
== See Also ==
 
== See Also ==
 
* [[Prefetch]]
 
* [[Prefetch]]
 +
* [[ReadyBoost]]
 +
* [[ReadyBoot]]
 +
* [[Windows SuperFetch Format|SuperFetch Format]]
 +
* [[Windows]]
  
 
== External Links ==
 
== External Links ==
* [https://googledrive.com/host/0B3fBvzttpiiSbl9XZGZzQ05hZkU/Windows%20Prefetch%20File%20(PF)%20format.pdf Windows Prefetch File (PF) format], by the [[libssca|libssca project]]
+
* [http://technet.microsoft.com/en-us/magazine/2007.03.vistakernel.aspx Inside the Windows Vista Kernel: Part 2], by [[Mark Russinovich]], March 2007
* [http://bitbucket.cassidiancybersecurity.com/prefetch-parser/wiki/Home Windows Prefetch file format], by the [http://bitbucket.cassidiancybersecurity.com/prefetch-parser prefetch-parser] project.
+
* [http://download.microsoft.com/download/7/E/7/7E7662CF-CBEA-470B-A97E-CE7CE0D98DC2/Win7Perf.docx Performance Testing Guide for Windows], by [[Microsoft]], August 18, 2009 
 +
* [http://en.wikipedia.org/wiki/Windows_Vista_I/O_technologies#SuperFetch Wikipedia: Windows Vista I/O technologies - SuperFetch]
 +
* [http://channel9.msdn.com/showpost.aspx?postid=242429 Channel 9 Interview with Michael Fortin of Microsoft on SuperFetch]
 +
* [http://www.informationweek.com/news/showArticle.jhtml?articleID=196902178 Microsoft Predicts The Future With Vista's SuperFetch] from Information Week
 +
* [http://jessekornblum.com/presentations/dodcc08-2.pdf DC3 Presentation: My You Look SuperFetching], by Jesse Kornblum
 +
 
 +
== Tools ==
 +
=== Open Source ===
 +
* [https://code.google.com/p/rewolf-superfetch-dumper/ rewolf-superfetch-dumper]
  
[[Category:File Formats]]
+
[[Category:Windows]]

Revision as of 07:10, 28 June 2014

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Further information might be found on the discussion page.

SuperFetch is a performance enhancement introduced in Microsoft Windows Vista to reduce the time necessary to launch applications. SuperFetch works with the memory manager service in Windows to analyze memory usage patterns over time to determine the optimal memory content for a given user for a date or time of day. This differs from the Prefetch technique used in Microsoft Windows XP, which preloads data into memory without analyzing usage patterns.

From [1]: SuperFetch prioritizes the following kinds of pages to remain in memory:

  • Pages of applications that are used most frequently overall.
  • Pages of applications that are commonly used when resuming:
    • After extensive hibernation (for example, first thing in the morning).
    • After shorter periods of sleep or hibernation (for example, after lunch).

If SuperFetch detects that the system drive is a fast SSD (as measured by Windows Experience Index Disk score), then SuperFetch turns off ReadyBoot, ReadyBoost, and the SuperFetch service itself.

Components

Robust performance

Robust performance (or robustness) is a component of SuperFetch to watch for specific file I/O access that might harm system performance by populating the standby lists with unneeded data.

Scenarios

SuperFetch distinguishes between different scenarios to accurately measure performance.

Cold scenario

In a cold scenario, the test applications are not already in memory when the test begins. Cold scenarios measure performance either after a state transition, such as boot or resume from hibernation, or after another application claims most of the available memory, such as after launching and quitting a game.

Warm scenario

In a warm scenario, some or all the scenario contents are in memory before measurement. This usually means that the test has run at least once during this logon session.

Configuration

Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the Registry value [2]:

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
Value: EnableSuperfetch

A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, services.msc [3].

File Formats

Data for SuperFetch is gathered by the %SystemRoot%\System32\Sysmain.dll, part of the Service Host process, %SystemRoot%\System32\Svchost.exe, and stored in a series of files in the %SystemRoot%\Prefetch directory [4]. These files appear to start with the prefix Ag and have a .db extension. Note that there are likely more SuperFetch database files named differently, presumably all using the .db extension.

The format of the SuperFetch database files is not fully known, there is available unofficial partial specification [5] and open source (GPL) dumper for .db files [6]. For more information see SuperFetch Format.

The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed [7].

The SuperFetch service is managed by the File Information FS MiniFilter service. It appears that most of the SuperFetch database files are updated (written) when the service is shut down. AgAppLaunch.db is also written when the service starts.

See Also

External Links

Tools

Open Source