CPU - (CPU|Processor) Time Counter

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1 - About

Cpu time

The count of cycles, also known as clockticks, forms the basis for measuring how long a program takes to execute.

See: Time - System Time (Sysdate)


3 - Definition

How do you measure Execution Time?

<MATH> \begin{array}{rll} && \text{Kernel} && \text{Time} \\ + &&\text{User} && \text{Time} \\ \hline \\ && \text{Total} && \text{Time} \end{array} </MATH>

CPU Time Running which code
Kernel (Sys) Time Operating system code - the time of CPU spent on the kernel (system) code only within the given process - other processes and the time when our process is blocked are not included.
User Time Time CPU spends running the program code (ie CPU time spent on non-kernel (user-mode) code only within the given process – so other processes and the time when our process is blocked are not included.).
Total time User + Kernel
Real Time Wall clock (total elapsed time) (can be lower than total time because of parallelism or longer because of wait) - user-perceived time it took to execute the command – from the start to the end of the call, including time slices used by other processes and the time when our process is blocked (e.g. I/O waiting)

4 - Process Explorer

CPU Time in process explorer

The performance tab of a process in Process Explorer. Red in the CPU usage graph indicates CPU usage in kernel-mode whereas green is the sum of kernel-mode and user-mode execution.

A data collector set can be configured via logman.exe to log the “% Processor Time” counter in the “Processor Information” object for this purpose.

5 - Formula

Every conventional processor has a clock with a fixed cycle time (or clock rate). At every CPU cycle, an instruction is executed.

<MATH> \text{CPU Time} = \text{CPU cycles executed} * \text{Cycle times} </MATH>

<MATH> \text{CPU cycles} = \text{Instructions executed} * \text{Average Clock Cycles per Instruction (CPI)} </MATH>

Putting it all together: <MATH> \text{CPU Execution Time} = \text{Instructions count} * \text{CPI} * \text{Clock Cycle Time} </MATH> where:

<MATH> \begin{array}{rll} \text{Instructions Count} && = && \frac{\displaystyle Instructions}{\displaystyle Programs} \\ \text{CPI} && = && \frac{\displaystyle Cycles}{\displaystyle Instruction} \\ \text{Clock Cycle Time} &&= && \frac{\displaystyle Seconds}{\displaystyle Cycle} \end{array} </MATH>


  • CPI is somewhat artificial (computed from the other numbers using) but it's seems to be intuitive and useful.
  • Use dynamic instruction count (#instructions executed), not static (#instructions in compiled code)

6 - Performance

Response time distributed through the request lifetime

6.1 - Busy vs Wait

Percentage shown in monitoring tool is generally: <MATH> \text{CPU Busy vs Wait Event} = \frac{\text{User} + \text{Kernel}}{\text{Wallclock Time}} </MATH>

6.2 - CPU Performance

<MATH> \text{CPU Performance} = \frac{1}{\text{Total CPU Time}} </MATH>

6.3 - System Performance

<MATH> \text{System Performance} = \frac{1}{\text{Wallclock Time}} </MATH>

7 - Units (Mhz to ns)

  • Rate often measured in MHz = millions of cycles/second
  • Time often measured in ns (nanoseconds)

<MATH> \begin{array}{rl} {X} \mbox{ MHz } && = && \frac{\displaystyle 1000}{\displaystyle X} \mbox{ ns} \\ 500 \text{ MHz} && \approx && 2 \text{ ns clock} \end{array} </MATH>

8 - Documentation / Reference

counter/resource/system/cpu/time.txt · Last modified: 2019/01/05 19:44 by gerardnico