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When to set the 3/GB switch

Posted on 2007-12-04
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After reading about the 3/GB switch option in Windows I am still unsure of when to set it.  It describes the allocation of the 4/GB of virtual address space between apps and the kernal.  Does the 3/Gb switch have anything to do with the amount of physical RAM in the PC?  I just increased the amount of RAM in a PC to 2.5/GB so that I can run Adobe CS3, this is when I began reading about the switch.  I always thought it was related to physical RAM not virtual memory.

Any help would be appreciated.

bshort1023
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by:lcit
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I've never seen it used outside of Windows Server envirnoments, but here's some info:

http://support.microsoft.com/kb/833721
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Suroj Shrestha earned 63 total points
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Chk this:
http://blogs.msdn.com/oldnewthing/archive/2004/08/05/208908.aspx

Go thru the comments as well, i hope that will clear your confusion.
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by:garycase
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The /3GB switch has NOTHING to do with how much physical memory is in the machine.   It simply changes the allocation of the virtual address space so that individual processes can have up to 3GB of virtual space instead of the 2GB they get by default.   This is true whether your system has 256MB of RAM installed or 4GB of RAM installed.

Even if you use /3GB and have an application which will take advantage of the additional addressing (such as CS3), the added paging activity will likely offset any gains from providing the added address space to the application.   Particularly if you only have 2.5GB of memory installed.

Bottom line:   In the situation you've described, I would NOT use /3GB unless you're doing things in CS3 which result in "out of memory" errors.
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by:bshort1023
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simcasuro: Good article\thread.

qarycase:  You say the 3GB switch has nothing to do with the amount of physical RAM but you later say the additional paging activity would offset any gains Particularly if you have only 2.5GB of RAM.  That implies to me that the 3GB switch might be worthwhile if I had more RAM.  Is that true?


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by:garycase
garycase earned 62 total points
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My comment simply meant that with only 2.5GB installed, any process that was allowed to have 3GB of address space would clearly require a fair amount of paging => since it could never have 3GB of "real" memory assigned.   It's true that if you had enough memory for the OS to keep its resident processes in memory and still have 3GB to assign to another process [i.e. if you installed 4GB and your system configuration let you "see" perhaps 3.25GB of it -- you'll never actually "see" 4GB for a variety of reasons] then a single process that was able to use 3GB MAY get some benefit from the /3GB switch.  Note that if your system doesn't "see" enough above 3GB (with 4GB installed) ... which depends on your specific configuration ... then the OS still won't be able to actually provide 3GB of "real" memory to any process.

But as I noted earlier, it's relatively rare to see any benefit from /3GB  [although CS3 IS one of the memory-hogging applications where it may help].    I wouldn't bother, however, unless you get "out of memory errors" in CS3.
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