Virtual Memory

Microsoft is taking tighter control of graphics memory with it's new driver model, and thus is able to provide virtual memory support for the graphics memory subsystem. What this means is that games no longer need to worry about running out of graphics memory. When software needs to write something to local memory, and local memory is full, Windows will be able to kick out something off the graphics card and put it in system memory (this is called paging) until it is needed. This happens without the software's intervention or knowledge. If system memory becomes full, data will be kicked out to the hard drive. Of course, if something like this happens the performance will definitely suffer.

Virtual memory isn't as much a performance enhancing tool as it is a way to remove the burden on the developer to manage memory usage around a hard limit of available space. Certainly, lots of paging will degrade performance, but lower performance is generally better than a crash. On the flip side, it is possible that virtual memory could increase performance by effectively replacing local graphics memory size with unused PCIe bandwidth. This has been the idea behind TurboCache and HyperMemory, but with the added advantage that the graphics driver doesn't need to worry about object or texture management between local and system memory.

Engineers have been wanting to see virtualized graphics memory for years, as operating on really huge data sets is made significantly easier when the software developer doesn't have to manage moving data in and out of graphics memory by hand. We've seen some limited benefits of utilizing both local and system memory on low memory TurboCache and HyperMemory cards. With game developers reaching towards ever larger data sets, high end parts will soon begin to benefit from virtualized graphics memory as well. Building the hardware to accommodate the possibility of higher latencies due to paging and allowing the OS to manage all the memory in the system will definitely help developers focus on building better games rather than better memory managers. That's not to say that memory management won't still be important to game developers. Making sure space and bandwidth are used efficiently are important factors in performance, but the ability to forget about hard limits in local memory will make it easier to take one efficient approach regardless of onboard memory.

Hardware Virtualization

Lately, all the big boys of computing have been infatuated with the idea of virtualization. It makes a whole lot of sense, really. With the advent of multi-core CPUs, AMD and Intel need to find ways to take full advantage of their processing power. Single thread execution time will never disappear as a factor in computing, and some algorithms just can't be parallelized.

Obviously, encouraging users to multitask is a simple way to provide a benefit to multi-core computing. The next step is to encourage developers to write highly multithreaded applications. Beyond that is to allow the user to run multiple operating systems on one set of hardware. One example of how this may be beneficial is in the use of a single system as a normal PC during its use as a home theater / DVR box. Another example is one we've already seen: Mac users running both Windows and OS X on Intel based Macs using a virtual machine manager like Parallels.

In order to really achieve the capabilities hardware providers would like to promote, more work must be done by hardware, software, and operating system providers. One of the major advances necessary is the virtualization of the graphics subsystem. With DirectX10 and the new WDDM (Windows Display Driver Model), graphics hardware is required to support virtualization. This is not a simple request, as games will no longer be guaranteed exclusive access to the hardware while running. We can potentially share game rendering with something like physics calculations on the same GPU. Or we could run a Folding@Home GPU client in the background while we play a game. On the extreme, multiple full screen 3d applications could be running concurrently.

Drivers and hardware will have to support context switching on a massive scale due the huge number of pipelines and registers supported in DX10 class hardware. With the advent of features like TurboCache and HyperMemory (and now graphics memory virtualization), hardware developers are already prepared to handle much larger latencies than we've seen in the past. The ability to preempt a process on the GPU will only increase the potential latency that will need to be addressed.

This is another major step in bringing the GPU closer in functionality to the CPU. More attention must be paid not only to instruction and thread scheduling, but the scheduling of multiple programs. This is no small task when such a high number of pipelines need to be managed. We are very interested in discovering how well NVIDIA has implemented this feature, but we won't be able to test this until we have access to an operating system, API, and software that support it as well.

Index All GPUs are Created Equal: Say Goodbye to Cap Bits
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  • aweigh - Friday, November 10, 2006 - link

    You can just use the program DX Tweaker to enable Triple Buffering in any D3D game and use your VSYNC with negligable performance impact. So you can play with your VSYNC, a high-res and AA as well. :) Reply
  • aweigh - Friday, November 10, 2006 - link

    I'm gonna buy an 88 specifically to use 4x4 SuperSampling in games. Why bother with MSAA with a card like that? Reply
  • DerekWilson - Friday, November 10, 2006 - link

    Supersampling can make textures blurry -- especially very detailed textures.

    And the impact will be much greater with the use of longer more detailed pixel shaders (as the shaders must be evaluated at every sub-pixel in supersample).

    I think transparency / adaptive AA are enough.

    On your previous comment, I don't think we're to the point where we can hit triple buffering, vsync, high levels of AA AND high resolution (2560x1600) without some input lag (triple buffering plus vsync with framerates less than your refresh rate can cause problems).

    If you're talking about enabling all these options on a lower resolution lcd panel, then I can definitely see that as a good use of the hardware. And it might be interesting to look at more numbers with these type of options enabled.

    Thanks for the suggestion.
    Reply
  • aweigh - Saturday, November 11, 2006 - link

    I never knew that about SuperSampling. Is it something similar to Quincux blurring? And would using a negative LOD via RivaTuner/nHancer counteract the effect?

    How about NVIDIA's Digital Sharpness setting in Color Correction? I've found a smidge of sharpening can do wonders to improve overall clarity.

    By the way, when you said Adaptive AA, were you referring to ATI cards?
    Reply
  • Unam - Friday, November 10, 2006 - link

    Derek,

    Saw your comment regarding the rationale for the test resolution, while I understand your reasoning now, it still begs the question how many of your readers have 30" LCD flat panels?
    Reply
  • DerekWilson - Friday, November 10, 2006 - link

    There might not be many out there right now, but it's still the right test platform for G80. We did test down to 1600x1200, so people do have information if they need it.

    But it speaks to who should own an 8800 GTX right now. It doesn't make sense to spend that much money on a part if you aren't going to get anything out of it with your 1280x1024 panel.

    Owners of a 2560x1600 panel will want an 8800 GTX. Owners of an 8800 GTX will want a 2560x1600 panel. Smooth framerates with the ability to enable 4xAA in every game that allowed it is reason enough. People without a 2560x1600 panel should probably wait until prices come down on the 8800 GTX or until games that are able to push the 8800 GTX harder to buy the card.
    Reply
  • Unam - Tuesday, November 14, 2006 - link

    Derek,

    A follow up to testing resolutions, the FPS numbers we see in your articles, are they maximum, minimum or average?
    Reply
  • Unam - Friday, November 10, 2006 - link

    Who the heck runs 2560x1600? At 4XAA? Come on guys, real world benchmarks please! Reply
  • DerekWilson - Friday, November 10, 2006 - link

    we did:

    1600x1200, 1920x1440, and even 1280x1024 in Oblivion
    Reply
  • dragonsqrrl - Thursday, August 25, 2011 - link

    ....lol, owned. Reply

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