The latest beta version of SteamOS, update 3.8.20, includes significant performance enhancements for devices equipped with discrete GPUs that have limited video RAM (VRAM). This update addresses issues related to graphics memory allocation and management within the Linux operating system, a collaborative effort involving independent contractors and developers from Valve, Intel, and Red Hat.
Understanding VRAM Allocation Challenges
Discrete GPUs typically have dedicated VRAM. However, systems also utilize a portion of the main system memory, known as the Graphics Translation Table (GTT), which the GPU can access via the slower PCIe interface. When a GPU needs more memory than is available in its dedicated VRAM, the operating system manages this by moving data between VRAM and GTT. This process, known as eviction, can lead to performance degradation and stutters if the OS incorrectly prioritizes which data to keep in VRAM or if frequent data shuffling occurs.
The Cgroup Solution
A key issue was that the operating system lacked a robust strategy to ensure that critical game data remained in the faster VRAM. To resolve this, developers implemented a solution using Linux control groups (cgroups). This feature allows for fine-grained control over resource allocation. By creating a specific VRAM cgroup controller, the system can now better manage memory, prioritizing game data and preventing less important data from occupying valuable VRAM. This prevents the OS from unnecessarily evicting data from VRAM to make space in GTT, ensuring more of the GPU's needs are met by the dedicated memory pool.
Performance Improvements Observed
Testing with demanding titles like Cyberpunk 2077 on a GPU with 8GB of VRAM demonstrated notable improvements. Prior to the update, the system used a significant portion of GTT even when VRAM was not fully utilized by the game. Following the implementation of the VRAM cgroup, Cyberpunk 2077 utilized more of the dedicated VRAM, and GTT usage dropped considerably. This more efficient memory management translates to a reduction in game stutters and performance pauses.
Impact on Different Devices
While this update offers substantial benefits for devices like the Steam Machine that utilize discrete GPUs, the impact on devices with integrated GPUs (iGPUs), such as the Steam Deck, is expected to be minimal. Integrated graphics share system memory, meaning the distinction between VRAM and GTT is less pronounced. While some minor gains might be realized from improved memory pointer management, significant performance boosts are unlikely for iGPU-based systems. The improvements are primarily aimed at enhancing the experience on hardware configurations with distinct, dedicated graphics memory.