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I'm looking at the output of top to view my system's load average. I want to understand whether the load average is reporting processes just waiting for CPU, or whether it's reporting processes that are waiting for either CPU or IO.

This article suggests that most unix systems report only CPU in their load, but that linux in particular reports both CPU and IO.

I'm guessing that OS X probably does not include IO in its load reporting, but I'd like to know for sure.

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On OS X, the kernel task accumulates IO wait time and the process accumulates CPU time managing the IO call setup and results but not the actual IO wait.

Put another way, in top or Activity Monitor, IO waiting is time when the CPU is idle. The system/kernel would take any core storage / filevault processing time needed to accomplish the IO but for the general case with a normal drive there isn't CPU overhead on IO past the normal virtual memory overhead.

  • that sounds like no? – emmby Nov 6 '12 at 17:58
  • Right. Waiting on I/O doesn't consume CPU time. – nohillside Nov 6 '12 at 18:00

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