Using more than 2GB Memory in Windows

[Anonymous user]

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32-bit versions of Windows are rare these days, this article is still here in case you are running old operating systems. On 64-bit versions of Windows, 32-bit processes can address the full 4GB of memory

Windows XP and 2003 32-bit operating systems can only address 4GB of RAM in total. By default, 2GB is reserved for user processes and 2GB for kernel usage. Normally, when a process reaches 2GB of memory used (2048MB, or roughly that number), the next request for memory will be refused. This will have undesirable effects in ClearSCADA leading to unexplained events such as server failures. In this scenario, an exceptions log will not be created as this process requires memory that is not available.

Windows (XP, 2003) can be set to change the 2GB user limit to 3GB, at the sacrifice of available kernel memory (2GB -> 1GB).

To do this, modify the boot.ini file (in the root directory of C drive) to have a /3GB switch for the operating system start up line (see http://support.microsoft.com/kb/833721) and reboot. Note that Windows hides the boot.ini file by default as a protected system files. This setting can be changed in the Folder Options of Windows Explorer.

An example file is:

[boot loader]
timeout=30default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS[operating systems] multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003, Enterprise" /noexecute=optout /fastdetect /3GB

The 32-bit process also needs to have a special flag set in it that allows it to use this feature of Windows. This feature is available from ClearSCADA 2005 R0 onwards. To view the amount of memory available to the ClearSCADA server, view the "Available Virtual Memory" setting on the Info tab of the ClearSCADA Server Status dialog.

In rare cases, networking problems have been seen on systems with the /3GB set. This has been seen as unexplained network-connected driver channels and SNMP points failing within ClearSCADA, and seems to be due to less kernel memory being available to Windows (1GB instead of 2GB). Boot.ini can be modified again to add the "/Userva=3030" switch, which allows more memory for Windows to assign to network resources, relieving the problem.

See http://support.microsoft.com/?kbid=810371 for more information.

This would make the above line in the file:

multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003, Enterprise" /noexecute=optout /fastdetect /3GB /Userva=3030

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