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Q171793: Information on Application Use of 4GT RAM Tuning

Article: Q171793
Product(s): Microsoft Windows NT
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbhowto
Last Modified: 10-AUG-2001

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The information in this article applies to:

- Microsoft Windows NT Server, Enterprise Edition version 4.0 
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SUMMARY
=======

This article describes the 4GT RAM Tuning feature of Windows NT Server
Enterprise Edition (Windows NT Server/E) version 4.0. With standard Windows NT
Server, the per-process address limit is 2 gigabytes (GB) of random access
memory (RAM). The 4GT feature of Windows NT Server/E increases this limit to 3
GB without introducing new APIs. 4GT does this by reducing the potential RAM
allocated to the Windows NT kernel from 2 GB to 1 GB.

Much of the information in this article was extracted from the Release Notes for
the product.

MORE INFORMATION
================

Applications developed for the Windows NT Server platform continue to grow, both
in terms of size and performance demands. For applications that are I/O
intensive, such as database management systems (DBMS), the use of a larger
process space can provide considerable performance benefits as time- intensive
I/O access to media is reduced. With the current Windows NT Server product, the
per-process address limit is 2 GB. 4GT increases this limit to 3 GB without
introducing new APIs. 4GT does this by reducing the potential RAM allocated to
the Windows NT kernel from 2 GB to 1 GB.

This feature benefits applications that run on powerful computers with more than
2 GB of physical RAM and that can take advantage of a larger address space. The
impact on developers and applications are summarized below.

Windows NT Server/E supports 4GT on Intel architecture servers only.

NOTE: The 4GT works only on 32-bit processors, so it does not increase the
addressable memory of Windows NT Server beyond its current limit of 4 GB.

Writing Applications for 4GT
----------------------------

User-mode Address Selection:

When 4GT is enabled, the highest bit of a virtual address cannot be used to
differentiate user-mode addresses from kernel-mode addresses.

Memory Allocation Issues:

Some dynamic link library (DLL) files load near the 2 GB boundary; therefore,
there is a region of the 2 GB space in which contiguous memory cannot be
allocated using VirtualAlloc.

Effects Visible in Kernel Mode:

Kernel-mode code can no longer assume the user/kernel boundary is at 0x80000000
or at any other number. Code that uses ProbeForRead or ProbeForWrite macros must
be rebuilt using new headers that no longer contain assumptions about kernel
space starting at 0x80000000.

Enabling 4GT Support in Your Applications
-----------------------------------------

The changes to support 4GT are done at both the system and application levels.

System Changes:

After you have installed Windows NT Server/E, you must modify the Boot.ini file
to enable 4GT. To enable 4GT, simply add the /3GB parameter to the startup
line.

NOTE: This change is only effective if you are using Windows NT Server/E. On
standard Windows NT, this flag will relocate the kernel, but applications will
be unable to access more than 2 GB.

For example:

  [boot loader]
  timeout=30
  default=multi(0)disk(0)rdisk(0)partition(2)\WINNT
  [operating systems] multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows NT
  Server Version 4.00" /3GB
  multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows NT Server Version 4.00 [VGA
  mode]" /basevideo /sos

NOTE: Some of the lines above have been wrapped for readability.

Application Changes:

No new APIs are required for 4GT support. Instead, memory allocations remain the
same, with the exceptions that are noted below in "Tips for Effective 4GT
Support." However, it would be ineffective for every application to
automatically be provided with a 3-GB address space. To provide a selective use
of 4GT, the following mechanism has been implemented:

- Executables that must see the 3-GB address space are required to have the bit
  IMAGE_FILE_LARGE_ADDRESS_AWARE set in their image header. This can be done
  using the Imagecfg tool that is included in the Support folder on the Windows
  NT Server/E compact disc. For example, to modify the target file DBMSApp.exe,
  type the following at a command prompt:

  Imagecfg -l DBMSApp.exe

NOTE: The linker also has a new switch (/ LARGEADDRESSAWARE) to link executables
with the IMAGE_FILE_LARGE_ADDRESS_AWARE bit. Setting this bit and then running
the application on a system that does not have 4GT support should not affect the
application.

Tips for Effective 4GT Support
------------------------------

The following guidelines are intended as a summary of changes that developers
will want to review when they consider enabling 4GT support within their
applications:

- Use GlobalMemoryStatus to get the amount of total user virtual space. Avoid
  using hard-wired constant definitions such as "#define HIGHEST_USER_ADDRESS
  0xC0000000". Try to detect the real value at runtime.

- Avoid signed comparisons with pointers. Some applications might crash on a
  4GT-enabled system for just this reason. A condition such as "if (pointer
  > 40000000)" will be false for a pointer that is above 2 GB.

- Code using the highest bit to tag items (data value versus an address value)
  will fail. For example, a 32-bit word might be considered a user- mode
  address if below 0x80000000 and an error code if above. This does not work
  anymore.

Setting a Paging File Size
--------------------------

If you are using the 4GT option and have a system with greater than 3 GB of
physical memory, you may want to consider changing the default size of your page
file. Note that this is changed by using the System tool in Control Panel. In
Windows NT 3.51, the default pagefile size is generally 11 MB larger than
physical memory. In Windows NT 4.0, the default pagefile size is equal to the
amount of physical memory on the system. On a 4-GB computer, this would result
in a 4.01-GB paging file. Under such circumstances, the effectiveness of this
paging file could be minimal, based on the total size, which is 4 GB.
Accordingly, a 256-MB page file could be a more effective use of disk space.
However, having a smaller paging size will affect the total memory commit size
for applications. You should review these settings with the Independent Software
Vendor (ISV) for your applications that are being run with the 4GT option.

Additional query words:

======================================================================
Keywords          : kbhowto 
Technology        : kbWinNTsearch kbWinNT400search kbWinNTSsearch kbWinNTSEntSearch kbWinNTSEnt400 kbWinNTS400search
Version           : winnt:4.0
Issue type        : kbinfo

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