/3GB on Enterprise server with 16GB of RAM

Posted on 2009-04-20
Last Modified: 2013-12-01
I've read a lot on the /3GB and /PAE switches over the past few hours (all the posts on this site included) but I'm feeling like I still don't quite understand how /3GB affects a server with more than 4GB of RAM.

Windows 2003 Server Enterprise Edition
16GB of RAM

From my understanding, the /PAE switch is required for the Operating System to make use of more than 4GB in a 32-bit environment. (increasing it to 36-bit) That being said, I do have /PAE enabled and the server sees all 16GB of physical memory.

Now everything I have read about /3GB uses a system with 4GB of physical memory as an example. So each user-process gets 2GB for the application and 2GB for the system without /3GB enabled. 3GB for the application and 1GB for the system when it is enabled.
How does this aspect work in my 16GB environment. Does each user process only ever use 4GB regardless of the total physical memory on the system? Would /3GB even benefit me since this server already has a large amount of memory? Am I forced to use /3GB if I want to permit the application to use more than 2GB of memory?

This particular server is used to run the single memory intensive application SAS Analytics Professional. Within this applications config file, I am able to designate how much memory it can use to perform its functions.

Thanks, in advance.
Question by:corphealth
    LVL 18

    Accepted Solution

    I think you need re-read the MS article about /PAE and /3G

    for any 32bits application, it only have 4G of address space. So no matter how much physical memory (over 4G) you haev, it will only use up to 4G. But whether that 4G is physical memory or page file (which is hard drive), the application itself doesn't know (and doesn't care), OS handles it. If you have /3G turned on, the application can have up to 3G of stuff in actual physical memory, instead of page in and out using paging file.
    Whether the /3G switch will benefit you or not, depends on the application. You mentioned it is running memory-intensive app, so it should, because by doing less page in and out, your performance should be better.

    Author Comment

    So the extra RAM I have over 4GB doesnt really benefit me unless i'm running multiple instances of the same application?
    All assuming the PF is never used...
    Instance 1 - 2GB application, 2GB system = 4GB = 4GB total
    Instance 2 - 2GB application, 2GB system = 4GB = 8GB total
    Instance 3 - 2GB application, 2GB system = 4GB = 12GB total
    Instance 4 - 2GB application, 2GB system = 4GB = 16GB total
    8GB total to application, 8GB total to system.
    Or with /3GB on...
    Instance 1 - 3GB application, 1GB system = 4GB = 4GB total
    Instance 2 - 3GB application, 1GB system = 4GB = 8GB total
    Instance 3 - 3GB application, 1GB system = 4GB = 12GB total
    Instance 4 - 3GB application, 1GB system = 4GB = 16GB total
    12GB total to application, 4GB total to system.
    Is this right?
    LVL 18

    Expert Comment

    in principle, that is correct.

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