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Intel's Sandy Bridge Graphics Architecture
By: David Kanter | 08-08-2011
Heterogeneous Integration

The graphics integration in Sandy Bridge is particularly novel as Intel is sharing the LLC with the GPU. The driver allocates regions of the cache at way granularity (128KB) – and can actually request the whole cache. Each thread can spill 32KB of data back to the LLC, for a total of nearly 2MB in the larger 12 shader core variants. Almost any GPU data can be held in the LLC, including vertices, textures and many other types of state.

The Sandy Bridge LLC and ring interconnect can rapidly pass data from the GPU back to the CPU – AMD’s Fusion is a far higher performance GPU, but that particular style of communication is discouraged. Since the GPU has a weaker ordering model, a flush command is needed to force data to be written back to the LLC prior to the CPU reading it. The driver can also allocate a portion of the LLC as a non-coherent cache for display data and other uses. For example, the results of transcoding might be written out to the the non-coherent region.

While this excellent system integration promises many benefits, at present it is restricted mainly to multimedia workloads. For graphics, it is largely an academic advantage to any but Intel’s driver team. The GPU is exposed through graphics APIs; yet neither OpenGL nor DirectX programs can interact with coherent memory and bypass I/O copies (let alone use the LLC). AMD has introduced an OpenCL extension for a zero copy mechanism on Windows systems already, and presumably Intel will follow once they have OpenCL and DirectCompute capable hardware. Intel’s graphics driver can take advantage of fast CPU/GPU communication, but that is only because it has raw access to the GPU hardware. These advances pave the way for Ivy Bridge and certainly promise good things in the future, but also serve to point out some of the deficiencies in the current generation.