Removing the 2GB Barrier

As it turns out, it's possible and actually quite easy to move the 2GB barrier by increasing the size of the user space, but at the cost of reducing the size of the kernel space. Under Windows XP, this is the fabled "/3gb" switch for boot.ini, and for Windows Vista it's the "IncreaseUserVa" option in BCDedit. By using these options applications can use more than 2GB of virtual address space (generally up to 3GB), and ideally this would be the end of the article.

Unfortunately this is not the case as there are problems on both the application and kernel side of things. On the application side, a common poor programming practice has been to always assume that an application will only be dealing with 2GB of user space; code that makes this assumption will likely error if more than 2GB of user space is actually available. This is avoidable by following proper programming practices, but as a safety precaution even with additional virtual address space allocated to user space Windows still defaults to limiting an application to 2GB. Only finally, if an application indicates to Windows that it is capable of handling more than 2GB, via the "/LARGEADDRESSAWARE" flag, may it have access to any space above 2GB.

As for the kernel, having had up to half of its space taken away must now find a way to live in a smaller space. The (in)ability of any specific system/Windows configuration to deal with this is why the 3gb switch is considered dangerous, seldom recommended, and just generally a bad idea. The biggest culprit here is drivers that run in kernel space. Like applications, they may assume that there's an entire 2GB of address space to work with, except unlike applications this space gets smaller instead of bigger.

Windows' own memory needs can also cause problems with the reduced kernel space. As we mentioned before, space is required for the kernel to do a multitude of things, if a lot of space is required - video cards with a lot of memory are a particular offender here - then everything needing space may not fit in the kernel space. Because there are no strong safeguards against these conditions it may cause a failure to boot or system instability, especially if the culprit is a driver that is well enough behaved to boot. Many modern drivers from hardware vendors that deal with enterprise-level hardware are capable of handling this, many more consumer hardware drivers are not. Stability concerns are the number one reason that breaking the 2GB barrier on a 32bit version of Windows is not recommended.

There is also a second concern however: performance. While an individual application may benefit from more user space in which to work, the kernel now has less space to cache data (as non-obvious as this may seem given all the addresses are virtual) and this can in theory hurt performance. This condition is something we will take a look at in a bit.

A Primer on Windows’ Memory Management A Case Study: Supreme Commander
POST A COMMENT

69 Comments

View All Comments

  • instant - Saturday, July 21, 2007 - link

    Could the writer please detail what, exactly, is wrong with XP-64bit edition?

    Having run it since it was released and never had a problem with it, I would be interested in knowing what problems I should have experienced.

    Reply
  • BUL - Tuesday, July 17, 2007 - link

    A few things I've found about /3GB in XP...

    1) In terms of "consumer" OS's, XP Pro is the only one to support it. (XP Home and W2K Pro DO NOT!) Vista has a different method, outlined in the article.
    2) If you change boot.ini, make sure you add a "3GB" boot option to your operating systems, not just change the one entry to /3GB. If a new driver gets installed that doesn't like /3GB, you could be left without the ability to change boot.ini back. (I can't find any info on whether /3GB affects Safe Mode, however safe mode may run in "3GB" mode!) So, unless you have the ability to WRITE to NTFS partitions outside of Windows, you may be stuck if you don't. See http://www.gehrytechnologies.com/catia/catia/catia...">http://www.gehrytechnologies.com/catia/catia/catia... for how to do it...

    Now, personally, I feel that a limitation like memory shows the "men from the boys" in terms of programmers. Windows and "endless memory" has allowed for bloatware for far too long and allowed for sloppy, inefficient programming... Just think back how many games & other programs could run on an XT with 640KB of RAM because the programmers wrote highly efficient code. (Lotus 1-2-3 for DOS was written primarily in assembler; Quattro Pro could open even larger spreadsheets than 1-2-3 because it utilized pseudo-virtual memory).

    Now we've come full-circle into the world of Windows, and some of these programs now have to become more efficient. That's what programmers are paid to do...
    Reply
  • Anorax - Monday, July 16, 2007 - link

    On some of the larger maps in AW2 the game will occasional crash to desktop and leave an error window saying it has exceeded the 2GB memory space. Not that helpful but at least it tells you why it crashed. Reply
  • the Chase - Sunday, July 15, 2007 - link

    Thank you so much for this article. I had a post awhile back in the forums asking for help on why in Vista I couldn't even run PR for bf2 unless I turned down the texture setting to Medium.

    Never figured it out though people here did offer some good ideas for fixes. After reading this article I knew this is what was going on. I searched on the Net and found out how to use Visual Studio C++ to modify the Bf2 executable. Problem solved! (I have the 64 bit version of Vista).

    And to think I was ready(thinkeng about anyway) going to buy another 2 GB's of memory to try and cure the problem. Thanks again.

    P.S. I also did the same fix for XP pro 64 and now PR runs much smoother than before in that OS also.
    Reply
  • spookware - Friday, July 13, 2007 - link

    Just wanted to mention that the description of how windows uses memory is correct for XP and previous versions but it is not quite how windows vista works.

    Specifically, vista does not automatically take the upper 2GB of adress space for the kernel any more. It instead grows the kernel adress space downard on demand. Allthough the associated switches (/3G) have the same prupose in vista what they are setting is actually the upper maximum for the kernel adress space.

    Reply
  • Magumi - Friday, July 13, 2007 - link

    I guess this means that if I want to buy a computer to last me at least three or four years, I need to get 64bit Vista and 8GB of RAM and hope that drivers and incompatibilities get resolved soon. Reply
  • Greyhead - Friday, July 13, 2007 - link

    Ryan, great article. Some other posters mentioned the use of the /PAE on servers. I recently discovered that MS recommends NOT using the /3GB switch in conjunction with /PAE. I followed their advice on a large SQL server we have and saw immediate positive results. The problem with /PAE /3GB combination is that when the OS is limited to 1 GB, there is not enough room for the size of the heap needed to support the /PAE option! This can be viewed using performance monitor - selecting "memory" options and then viewing total PTEs available (Page Table Entries). There are MS articles that describe the minimum PTEs needed, and before I changed our server it was way under the minimum. We had stupid errors on the server - blue screens, "not enough memory" errors when transferring files to another machine. Once the change was made, these problems disappeared. 2003 server has a more tunable /3GB switch using the /USERVA switch. There are technet articles which provide guidance on it's usage.

    Keep up the great work -

    -bill
    Reply
  • redpriest_ - Friday, July 13, 2007 - link

    binary is illegal. Reply
  • Kougar - Friday, July 13, 2007 - link

    The moment I saw the title I was thinking of Supreme Commander. I particularly enjoy the insanely huge 8-player games, but it only took about 40 minutes before these would crash... oddly enough they did not always crash either in some situations, making the crashing appear random and only confusing my attempts to troubleshoot this game. It would have been quite appreciated that this issue would have at least been mentioned in the game's readme file, if nothing else.

    Having (very luckily) stumbled across MadBorris's thread I made the modifications to XP and SC has been running since. I have not run into any instability or issues with XP configured with the /3gb switch for what it is worth. Am I wrong in that users with video cards featuring smaller onboard memory sizes will have an "advantage" with this problem? There is a large difference between a 320mb GTS and 740MB GTX, or heaven forbid a 1GB HD 2900 card? And while on the subject does a dual-GPU configuration (and therefore dual VGA memory) make things even worse?
    Reply
  • Ryan Smith - Friday, July 13, 2007 - link

    Yes to the first question. To the second question I believe that's also a yes, but I don't have a system configured to test that theory. Reply

Log in

Don't have an account? Sign up now