PIM-VR: Erasing Motion Anomalies In Highly-Interactive Virtual Reality World With Customized Memory Cube
With the revolutionary innovations emerging in the computer graphics domain, virtual reality (VR) has become increasingly popular and shown great potential for entertainment, medical simulation and education. In the highly interactive VR world, the motion-to-photon delay (MPD) which represents the delay from users' head motion to the responded image displayed on their head devices, is the most critical factor for a successful VR experience. Long MPD may cause users to experience significant motion anomalies: judder, lagging and sickness. In order to achieve the short MPD and alleviate the motion anomalies, asynchronous time warp (ATW) which is known as an image re-projection technique, has been proposed by VR vendors to map the previously rendered frame to the correct position using the latest headmotion information. However, after a careful investigation on the efficiency of the current GPU-accelerated ATW through executing real VR applications on modern VR hardware, we observe that the state-of-the-art ATW technique cannot deliver the ideal MPD and often misses the refresh deadline, resulting in reduced frame rate and motion anomalies. This is caused by two major challenges: inefficient VR execution model and intensive off-chip memory accesses. To tackle these, we propose a preemption-free Processing-In-Memory based ATW design which asynchronously executes ATW within a 3D-stacked memory, without interrupting the rendering tasks on the host GPU. We also identify a redundancy reduction mechanism to further simplify and accelerate the ATW operation. A comprehensive evaluation of our proposed design demonstrates that our PIM-based ATW can achieve the ideal MPD and provide superior user experience. Finally, we provide a design space exploration to showcase different design choices for the PIM-based ATW design, and the results show that our design scales well in future VR scenarios with higher frame resolution and even lower ideal MPD.