What does CPU usage mean?

Question

What does it mean, when a CPU usage of a core is 100 percent?

Does it mean that a process has peaked the maximum possible performance of a core (with vectorization) or does it mean that the CPU is doing as many cycles as the core clock is?

Also, if it is 100 percent, can I imply that the process is CPU bound, i.e. a faster and bigger Cache, Memory, bus, etc. will not make any difference?

Answer 1

CPU usage usually means: What fraction of real time is the CPU doing any work. 90% CPU usage means that the CPU is 90% busy and 10% idle. When the CPU is idle it is doing nothing and it just waits for interrupts.

In this view the CPU always includes the caches and the memory. A bigger cache and a faster memory will always make your CPU work faster in this sense.

A CPU usage of 100% means that the processes are in sum CPU bound. They would run faster if the CPU would be faster.

A CPU usage of less than 100% means that the processes are in sum I/O bound. They would not run faster if the CPU would run faster.