iray rendering engine use gpu rendering which takes much less time than mental ray in rendering a large scene with a great amount of details and light bounces in GPU Photorealistic Rendering .
i remember when it was tested for comparison between cpu and gpu render times on gtx 590, Quadero k5000 and a core i7 980x .
although gtx 590 can use cuda cores for rendering , it becomes extremely hot and unstable unlike the Quadro which is meant to be used in long render times .
but gpu rendering is great to test scenes and adjust you regular Mental Ray renderings for a physical correct look, though using a rendering engine like mental ray or vray is somehow better because you can tweak the look and speed of renderings on how you want it .
CPU support a wide range of instruction, including vector instructions. Also CPU cores can handle any loops and conditions. Compared to that GPU-s are much more limited. They can't really use conditions affecting many type of loops, and if they run out of tasks, they can't proceed to the next steps - but they are massively more cores compared to CPU , take for instance the old gtx 590 vs the newest intel corei7 4960x .
a 1024 cuda cores vs 6 cores comparison , the numbers can tell .
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