Virtual Address Space Usage in Windows Game Development

Virtual Address Space Usage in Windows Game Development

August 2, 2007

Abstract

Windows VistaÐ'® significantly improved the way in which the operating system handles the graphic processing unit (GPU). The Windows Vista Display Driver Model (WDDM) enables better sharing of GPU resources among multiple graphics applications. The video memory manager component of WDDM virtualizes the available video memory resources for applications to use. This enables the new Windows Aero experience to run alongside other applications, such as video playback, which also use the GPU.

However, the virtualization of video memory increases the amount of virtual address space that applications use. Consequently, some applications may exhaust virtual address space when running on 32-bit Windows Vista. This paper describes the types of applications that might encounter such problems and the steps that Microsoft is taking to reduce virtual address space pressure for games running under WDDM.

This information applies for the Windows Vista operating system.

The current version of this paper is maintained on the Web at:

http://www.microsoft.com/whdc/device/display/WDDM_VA.mspx

References and resources discussed here are listed at the end of this paper.

Contents

Introduction 3

Virtual Address Space Limits in 32-Bit Windows 3

Solutions 4

Interim Solution 4

Long-Term Solution 4

Resources 5

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Introduction

Windows VistaÐ'® significantly improved the way in which the operating system handles the graphic processing unit (GPU). The Windows Vista Display Driver Model (WDDM) enables better sharing of GPU resources among multiple graphics applications. WDDM was designed to work with both the new MicrosoftÐ'® DirectXÐ'® 10 architecture and the existing DirectX 9 and older architectures. Existing DirectX games work with both the WindowsÐ'® XP Display Driver Model (XPDM) and WDDM drivers.

In Windows XP, video memory was allocated on a "first-come, first-served" basis. If a single application used all the available video memory, other applications were completely starved. In Windows Vista, however, the video memory manager component of WDDM virtualizes the available video memory resources for applications to use. Thus, video memory is available for multiple DirectX and Desktop graphics applications that execute simultaneously. Virtualization enables the new Windows Aero experience to run alongside other applications, such as video playback, that also use the GPU.

The virtualization of video memory in WDDM uses more virtual address space within an application’s address space than previous models. For applications that already approach the limits of their address space on 32-bit systems, running under WDDM can result in the exhaustion of the address space. This paper describes the virtual address space problem on a 32-bit Windows operating system and details the steps that Microsoft is taking to reduce virtual address space pressure for games running under WDDM.

Virtual Address Space Limits in 32-Bit Windows

On modern operating systems such as Windows, applications execute within their own private virtual address space. The size of the virtual address space is typically fixed at 2 GB for 32-bit applications. The amount of virtual address space is not related to the amount of physical memory on the machine.

Every memory allocation, file mapping, or library that is loaded by an application consumes space in this virtual address space. When the application consumes all of its virtual address space, any further such operations simply fail. Although all applications should be coded to handle memory allocation failures, many applications do not recover properly from such failures and consequently become unstable or crash.

Existing games and other graphics applications often allocate