Tile size is chosen so that the target surfaces in the RTset for that tile will all fit in a core’s L2 cache. Thus an RTset with many color channels, or with large high-precision data formats, will use a smaller tile size than one with fewer or low-precision channels. To simplify the code, tiles are usually square and a power-of-two in size, typically ranging in size from 32x32 to 128x128. An application with 32-bit depth and 32-bit color can use a 128x128 tile and fill only half of the core’s 256KB L2 cache subset.
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Figure 8 shows a back-end implementation that makes effective use of multiple threads that execute on a single core. A setup thread reads primitives for the tile. Next, the setup thread interpolates per-vertex parameters to find their values at each sample. Finally, the setup thread issues pixels to the work threads in groups of 16 that we call a qquad. The setup thread uses scoreboarding to ensure that qquads are not passed to the work threads until any overlapping pixels have completed processing.
The three work threads perform all remaining pixel processing, including pre-shader early Z tests, the pixel shader, regular late Z tests, and post-shader blending. Modern GPUs use dedicated logic for post-shader blending, but Larrabee uses the VPU.
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One remaining issue is texture co-processor accesses, which can have hundreds of clocks of latency. This is hidden by computing multiple qquads on each hardware thread. Each qquad’s shader is called a fiber. The different fibers on a thread co-operatively switch between themselves without any OS intervention. A fiber switch is performed after each texture read command, and processing passes to the other fibers running on the thread. Fibers execute in a circular queue. The number of fibers is chosen so that by the time control flows back to a fiber, its texture access has had time to execute and the results are ready for processing.
Keep in mind that IHVs have to also respect the threshold specific existing and future protocols will set; an external power supply for instance would be a ridiculous idea.
