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CPU Utilization is Wrong (2017)

3 days ago
https://www.brendangregg.com/blog/2017-05-09/cpu-utilization-is-wrong.html
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  • #System Monitoring
  • #Memory I/O
  • #CPU Performance
  • CPU utilization (%CPU) is misleading as it measures non-idle time, not actual processor busyness.
  • Modern CPUs often stall due to memory I/O, making high %CPU not indicative of CPU being the bottleneck.
  • Performance Monitoring Counters (PMCs) like IPC (Instructions Per Cycle) provide better insight into CPU performance.
  • An IPC < 1.0 suggests memory-bound workloads, while IPC > 1.0 indicates instruction-bound workloads.
  • Tools like `perf` and `tiptop` can measure IPC and stalled cycles, offering actionable tuning insights.
  • Cloud environments may limit access to PMCs, but some providers like AWS EC2 now support them.
  • Performance monitoring tools should include IPC or stalled cycles to avoid misleading users about CPU utilization.
  • Other factors like temperature, turboboost, and spin locks also contribute to misleading CPU utilization metrics.
  • Renaming %CPU to %CYC (cycles) could better reflect what the metric actually measures.
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