MULTICORES DON’T ADD UP

4. July 2006, 20:29 | by WD Milner | Full Article |

A discussion with a colleague over his selection of a new CPU and paraphenalia ended up in a short clarification of what dual-core, and indeed multi-core processors in general mean to computing speed.

AMD and Intel (and others) have had dual core procssors out for a while now, with quad-core reported to be available soon. But does a dual core 2 GHz processor really mean you have 4GHz of speed? And will quad cores mean 8GHz performance? As usually the answer is "maybe".

If you run many small programs that have little or no interaction with each other then it is indeed possible to acheive a performance boost close to that of the multiple times the clocked speed. If your application is large, however, and needs to be divided across cores, and the segments need to interact across cores, perhaps intensely as in video compression or encryption engines or streaming multimedia, then the picture changes significantly.

In such cases, you will not obtain full utilization of all cores, and figuring out exactly how much speed you will get can be difficult. Once you start to handle multicore chips, concepts such as idle and busy tend to blur and become hard to account for.

An idle core may be using resources on a competing basis with the other cores and therefore be hard to utilize; and if they are only idle for a few cylces they may actually be useless. One cannot simply add up all the free cycles regardless of their status or the state of other cores and factor them into "processing headroom". Determining how well multi-core processors perform is complex and requires much more than adding GHz

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Categories: ,
Keywords: core,processors,cpus,computing,chips

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