Well I'm still waiting on my copy of Vista Ultimate... I can't wait to jump in on the fun .
Among all of our tests, there are three identifiable patterns, all relating to some difference between Vista and XP. Those are:
1. Vista is using more address space than XP in all situations
2. The amount of address space used with Vista seems to be related to the amount of video memory on our video card
3. XP on the other hand does not fluctuate at all, the address space usage is the same no matter what card we use.
We suspect that the issue is related to some combination of how Vista handles video cards, and Vista being more resource-hungry as a side effect of it being a heavier operating system in general. With the latter generally being unavoidable and unchangeable, we're left with trying to figure out what about the former is having the greatest impact.
One of the most notable features introduced in the Windows Display Driver Model(WDDM) for Vista was virtualized memory, which allows for Vista to use the normal computer memory hierarchy as part of a memory hierarchy for video memory. The reason we mention this is that tucked away in one of Microsoft's documents on WDDM is a note with the following: "Memory allocations are limited to the application address space primarily for security reasons. WDDM provides increased security by isolating applications and their resources from each other." Our suspicion is that this feature is causing video memory to eat up more of the application's address space than under XP, which would explain why the extra address space is being used, why it's variable with video memory, and why XP doesn't show something similar.
Unfortunately we have been unable to find any confirmation that this is the case. When presented with the some of our numbers, only NVIDIA would comment, cryptically saying that "Microsoft is aware of the issue and they are working on it. More details will be provided as they become available." Whether this means that the problem is what we're guessing or something entirely different is not something we can get an answer at this point.
As far as Windows Vista is concerned, all of this weighs heavily on a fledgling operating system that many people are waiting to get its first service pack before trying. While the performance problems we saw earlier this year with Vista have largely cleared up, now it seems there's a definite issue with address space usage in conjunction with applications that make heavy use of a video card (e.g. games). In turn, it's hard to understate the situation, since hitting the 2GB barrier typically results in crashing.
The situation isn't entirely unmanageable, but the number of questions left is uncomfortably larger than the number of answers provided so far. The problem can be solved by users in several ways we've outlined in part 1 of this series, but none of these solutions are truly satisfactory. The real solution is going to have to come from Microsoft, who thankfully is aware of the problem and we presume we'll be hearing about a solution from them sooner than later. In the mean time however serious gamers will want to take a critical look at Vista if they are currently using it or are thinking about switching to it. This isn't a severe problem with so few serious gamers currently on Vista, but it may be best to hold off on switching to Vista until Microsoft has the issue sorted out.
On a lighter note, not everything we've seen today is bad news. The address space usage on XP is significantly lighter than Vista, making the situation far less dire. Instead of having to resort to any of the solutions we outlined in part 1, the majority of XP users should be able to get away with not needing to take any action at all, at least for the next few months. The 2GB barrier still exists and still will be a problem, but ideally by the time the situation is as bad on XP as it is on Vista today, we'll have a solution to this address space issue from Microsoft, along with a clearer path on the migration to 64bit.
And getting back to Supreme Commander for a moment, our problems have not fallen on deaf ears over at developer Gas Powered Games. They let us know that the next version of Supreme Commander, Forged Alliance, will feature several engine improvements related to the issues we've covered. They are switching memory allocators to an in-house solution which is lighter and uses less memory, and are also attempting to put in a notification system for alerting users if the game has run out of address space to work with. Finally the game will be shipping as large address aware and will be tested as such. Hopefully we'll see other developers taking similar steps over the next year.
We'll have more on address space usage in Windows Vista as we find out more.