Q: What makes the GPU so much faster than the CPU? A: The GPU is not faster than the CPU. CPU and GPU are designed with two different goals, with different trade-offs, so they have different performance characteristic. Certain tasks are faster in a CPU while other tasks are faster computed in a GPU. The CPU excels at doing complex manipulations to a small set of data, the GPU excels at doing simple manipulations to a large set of data. The GPU is a special-purpose CPU, designed so that a single instruction works over a large block of data (SIMD/Single Instruction Multiple Data), all of them applying the same operation. Working in blocks of data is certainly more efficient than working with a single cell at a time because there is a much reduced overhead in decoding the instructions, however working in large blocks means there are more parallel working units, so it uses much much more transistors to implement a single GPU instruction (causing physical size constraint, using more energy, and producing more heat). The CPU is designed to execute a single instruction on a single datum as quickly as possible. Since it only need to work with a single datum, the number of transistors that is required to implement a single instruction is much less so a CPU can afford to have a larger instruction set, a more complex ALU, and a more sophisticated caching schemes. Its instruction cycles is also faster. The reason why we are still using CPU is not because x86 is the king of CPU architecture and Windows is written for x86, the reason why we are still using CPU is because the kind of tasks that an OS needs to do, i.e. making decisions, is run more efficiently on a CPU architecture. An OS needs to look at 100s of different types of data and make various decisions which all depends on each other; this kind of job does not easily parallelises, at least not into an SIMD architecture. In the future, what we will see is a convergence between the CPU and GPU architecture as CPU acquires the capability to work over blocks of data, e.g. SSE. Also, as manufacturing technology improves and chips gets smaller, the GPU can afford to implement more complex instructions.
Posted on: Wed, 14 Aug 2013 13:50:34 +0000
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