Gregg: CPU Utilization is Wrong

Brendan Gregg asserts that CPU utilization is the wrong metric to be looking at when tuning a system. Much of the time when the CPU appears to be busy, it's actually just waiting for memory. "The key metric here is instructions per cycle (insns per cycle: IPC), which shows on average how many instructions we were completed for each CPU clock cycle. The higher, the better (a simplification). The above example of 0.78 sounds not bad (78% busy?) until you realize that this processor's top speed is an IPC of 4.0. This is also known as 4-wide, referring to the instruction fetch/decode path. Which means, the CPU can retire (complete) four instructions with every clock cycle. So an IPC of 0.78 on a 4-wide system, means the CPUs are running at 19.5% their top speed. The new Intel Skylake processors are 5-wide."

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