Virtual Address Space and OOMs

Virtual Address Space (VAS)

The Team is seeing an increasing number of posts at flight sim internet sites and support forums, including our own , on the subject of OOMs.  The majority of these errors occur in FSX and P3D, but FS9 can also be involved.

So what is an OOM and how does it happen?  First, we need to have a brief look at Windows memory management.  What follows is from https://msdn.microsoft.com/en-us/library/windows/desktop/aa366779(v=vs.85).aspx  There are more details there if you wish to immerse yourself in details.

Windows Memory Management

Each 32-bit process (FS9, FSX or P3D) on 64-bit Microsoft Windows has its own virtual address space that enables addressing up to 4 gigabytes of memory.  All threads of a process can access its virtual address space. However, threads cannot access memory that belongs to another process, which protects a process from being corrupted by another process.

A virtual address does not represent the actual physical location of an object in memory; instead, the system maintains a page table for each process, which is an internal data structure used to translate virtual addresses into their corresponding physical addresses. Each time a thread references an address, the system translates the virtual address to a physical address.

We recommend using Windows 7 64-bit as it provides the full 4GB block of VAS to FSX and P3D.   32-bit Windows provides only a maximum of 2GB of VAS.  32-bit Windows VAS can be increased to 3GB by using “the 3GB switch”.  See this post for more information:  http://ifly.flight1.net/forums/forum_posts.asp?TID=838&title=accessing-more-memory

Nonetheless, the result of the tweak is 1GB less VAS than with 64-bit Windows.  Therefore, OOMs and OS errors will be more likely.

An item to remember for the future:  a 64-bit process on 64-bit Windows has a virtual address space of 8 terabytes.

Another great source of information on this topic is Mark Russinovich’s blog: 
http://blogs.technet.com/b/markrussinovich/archive/2008/11/17/3155406.aspx

Implications for FS

Everything running as a part of Flight Simulator consumes part of the 4GB allocated to the process.  So all of that complex scenery, AI aircraft, clouds, high definition textures, etc., etc. along with complex aircraft models and the manner in which Flight Simulator and Windows handle memory can combine to exceed 4GB.  That’s when you see the infamous “Windows has run out of memory and will shut down” error message.

So what’s to do?  There are many discussions around the internet on this topic.  One of the more relevant may be seen at:

https://kostasfsworld.wordpress.com/fsx-oom-and-addon-vas-usage/

For P3D, http://www.robainscough.com/Prepar3D_Settings_2.html provides different settings to test.  P3D users should be aware that the simulation seems to be more sensitive to factors which cause OOMs.