Gregg: CPU Utilization is Wrong

Brendan Gregg assertsthat CPU utilization is the wrong metric to be looking at when tuning asystem. Much of the time when the CPU appears to be busy, it's actually just waiting formemory. "The key metric here is instructions per cycle (insns per cycle:IPC), which shows on average how many instructions we were completed foreach CPU clock cycle. The higher, the better (a simplification). The aboveexample of 0.78 sounds not bad (78% busy?) until you realize that thisprocessor'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 canretire (complete) four instructions with every clock cycle. So an IPC of0.78 on a 4-wide system, means the CPUs are running at 19.5% their topspeed. The new Intel Skylake processors are 5-wide."