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I was trying to figure out why my Mountain Lion server was running slow, and I when I opened Activity Monitor this is what I saw:

activity monitor shot

Question 1: how am I getting >400% of CPU on the summed CPU column, when I only have 4 cores available?

Question 2: given that I have all this CPU activity, how am I getting these usage reports in the lower-left corner, esp. the 38% idle one?

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up vote 4 down vote accepted

Check this out for a previous post. But, adding a bit to that answer...

First - while you have 4 physical cores, you have hyperthreading active, which allows each core to serve as 2 virtual cores. This will give you a theoretical maximum CPU usage of 100% x 8, or 800%.

Second - the % reported in the activity monitor is showing total CPU% (so it won't exceed 100%). Looking at the CPU numbers in your table, you're using a bit more than 485% out of the maximum of 800%. 485/800 is about 60%, which is your total usage reflected here...

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