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  • Windows 4GB limit

    I've been going round and round on the web trying to find out about the 4GB limit on Windows, but I'm not 100% having seen some arguments about what it means and if/how to get around it.

    I have an XP Pro workstation with 4GB ram in it, of which it can see around 3.5GB. I'd like to squeeze out the remaining half a gig since I'm running Virtual PCs on this box.

    I also have some 2003 Servers that may need more than 4GB of ram (Opterons running 32-bit 2003 Server).


    1. Does the /PAE boot switch only work on Intel processors or AMD as well?
    2. Does the /PAE boot switch work on XP Pro or just on Windows Servers?
    3. Does the /PAE switch allow you to use more than 4GB physical ram on a 32-bit OS?
    4. Can regular apps use more than the 4GB of ram if /PAE is used? (eg. Virtual PC, Vfox etc)

  • #2
    Re: Windows 4GB limit

    If you have 4GB or more of physical RAM in your server, use the /PAE switch in your boot.ini file. It's really as simple as that.

    XP doesn't support more than 4GB of RAM therefore the switch is not relevant.

    Think of 32 bit memory addressing like this:

    You hold in your arms a bag of groceries that represents 4GB of memory. That bag of groceries is all that you can carry, thus, your limited to handling 1 bag of groceries, or 4GB of memory at one time.

    Now consider that you have a cupboard that you can put groceries into and pull groceries out of. The cupboard is your extra stroage space and represents memory area above 4GB. In order to access the cupboard, you need to use the /PAE switch in the boot.ini. This allows you to take groceries out of your bag, put them in the cupboard, and grab other groceries out of the cupboard and put them in your bag.

    It's a bit like memory swapping to disk, except you swapping memory from your 4GB addressable memory space, to your extended memory above the 4GB memory limit.

    So all of your memory 4GB and above is "usable", however, only the bottom 4GB is addressable or usable at one time. Windows handles the process of moving the memory around so that it all gets used.

    The mere inefficiency and overhead involved in the smoke and mirrors process to access memory above 4GB is one of the attractive features of 64 bit platforms - they don't have the 4GB limitation like 32 bit platforms do. 64 bit platforms can access more memory than you can afford to buy.

    Jas
    Last edited by jasonboche; 8th June 2007, 19:14.
    VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+
    boche.net - VMware Virtualization Evangelist
    My advice has no warranties. Follow at your own risk.

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    • #3
      Re: Windows 4GB limit

      thanks I actually already knew that from hunting around, but have just confirmed that XP is indeed limited to 4GB even though it uses the /pae switch. 2003 standard is the same.

      this is quite lame and is really just a licensing issue rather than a technical limitation.

      M$ just want to charge more for Enterprise and Datacenter Editions.

      Comment


      • #4
        Re: Windows 4GB limit

        Also look at the /3GB switch. Seems to have mixed results but it may be worth a try.

        I'm wondering if your 3.5 GB is because of space allocated to the display adapter. You may want to experiment in the AGP Aperture size in the BIOS.

        Just a thought.
        Last edited by rvalstar; 8th June 2007, 18:54. Reason: typo
        Cheers,

        Rick

        ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

        2006-2099 R Valstar. This post is offered "as is" for discussion purposes only with no express or implied warranty of any kind including, but not limited to, correctness or fitness for use. Nothing herein shall be construed as advice. Attempting any activity based on information in this post is done at your own risk.

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        • #5
          Re: Windows 4GB limit

          I don't fancy killing my graphics to try to squeeze a few more megs but thanks for the tip.

          Comment


          • #6
            Re: Windows 4GB limit

            Originally posted by rvalstar View Post
            Also look at the /3GB switch. Seems to have mixed results but it may be worth a try.
            I was under the impression that the /3GB switch had to do with how much memory certain applications would reserve... (I could easily be mistaken)
            Regards,
            Jeremy

            Network Consultant/Engineer
            Baltimore - Washington area and beyond
            www.gma-cpa.com

            Comment


            • #7
              Re: Windows 4GB limit

              Originally posted by JeremyW View Post
              I was under the impression that the /3GB switch had to do with how much memory certain applications would reserve... (I could easily be mistaken)
              By default, of the 4GB of addressable memory space, Windows OS gets 2GB, applications get 2GB.

              If you apply the /3GB switch, Windows now gets only 1GB and applications get 3GB.

              Generally used with SQL and Exchange.

              With the /3GB switch comes some goofy nuiances, like, all of the sudden you have no memory to install trivial little applications like WinZip. Use it sparingly.
              VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+
              boche.net - VMware Virtualization Evangelist
              My advice has no warranties. Follow at your own risk.

              Comment


              • #8
                Re: Windows 4GB limit

                For Oracle, /3GB is a must. SQL Server is still a toy DB.

                Don't know how VM products consume memory. If they consume User vs. Kernel then the /3GB may help.

                I'd be interested to see what caused a system to consume 1 GB of kernel such that you couldn't install WinZip. Sounds like other factors contributed to that one.
                Cheers,

                Rick

                ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                2006-2099 R Valstar. This post is offered "as is" for discussion purposes only with no express or implied warranty of any kind including, but not limited to, correctness or fitness for use. Nothing herein shall be construed as advice. Attempting any activity based on information in this post is done at your own risk.

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                • #9
                  Re: Windows 4GB limit

                  Originally posted by rvalstar View Post
                  I'd be interested to see what caused a system to consume 1 GB of kernel such that you couldn't install WinZip. Sounds like other factors contributed to that one.
                  It was a SQL2000 cluster. And it wasn't just winzip, basically any setup.exe wouldn't run. Got a Not Enough Memory error. Workaround is to reboot the box into a non /3GB mode to install the application, then reboot back into /3GB mode.

                  Yeah, funky.
                  VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+
                  boche.net - VMware Virtualization Evangelist
                  My advice has no warranties. Follow at your own risk.

                  Comment


                  • #10
                    Re: Windows 4GB limit

                    I'm still playing "Devil's Advocate" here. I'm guessing I could tune down the SQL Server memory requirement and get the cluster to run successfully (not optimally) w/ 1 GB RAM total per server. I've done that both w/ SQL Server and Oracle 8i, 9i, 10g. Thus, I have to conclude either the /3GB switch does not work as expected (and M$ has banks of programmers that make that unlikely though not improbable) or something was peculiar to your installation ("Maximize data throughput for file sharing" when it should have been "Maximize data throughput for network applications" or ???)
                    Cheers,

                    Rick

                    ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                    2006-2099 R Valstar. This post is offered "as is" for discussion purposes only with no express or implied warranty of any kind including, but not limited to, correctness or fitness for use. Nothing herein shall be construed as advice. Attempting any activity based on information in this post is done at your own risk.

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                    • #11
                      Re: Windows 4GB limit

                      There exists an EXCELLENT EXCELLENT EXCELLENT colorful whitepaper on /PAE and /3GB. If anyone would like the link to it, let me know, I have it. Did I mention it was excellent?
                      VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+
                      boche.net - VMware Virtualization Evangelist
                      My advice has no warranties. Follow at your own risk.

                      Comment


                      • #12
                        Re: Windows 4GB limit

                        And you're not posting the link because ??? Hope you're not looking for Paris Hilton-like notoriety.
                        Cheers,

                        Rick

                        ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                        2006-2099 R Valstar. This post is offered "as is" for discussion purposes only with no express or implied warranty of any kind including, but not limited to, correctness or fitness for use. Nothing herein shall be construed as advice. Attempting any activity based on information in this post is done at your own risk.

                        Comment


                        • #13
                          Re: Windows 4GB limit

                          Because I don't have it right now. I have it at home.
                          VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+
                          boche.net - VMware Virtualization Evangelist
                          My advice has no warranties. Follow at your own risk.

                          Comment


                          • #14
                            Re: Windows 4GB limit

                            I liked the "celebutant" angle better than the "at work" bit.
                            Cheers,

                            Rick

                            ** Remember to give credit where credit is due and leave reputation points sigpic where appropriate **

                            2006-2099 R Valstar. This post is offered "as is" for discussion purposes only with no express or implied warranty of any kind including, but not limited to, correctness or fitness for use. Nothing herein shall be construed as advice. Attempting any activity based on information in this post is done at your own risk.

                            Comment


                            • #15
                              Re: Windows 4GB limit

                              Memory Management - Demystifying /3GB

                              http://blogs.technet.com/askperf/arc...fying-3gb.aspx
                              VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+
                              boche.net - VMware Virtualization Evangelist
                              My advice has no warranties. Follow at your own risk.

                              Comment

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