As a Performance Engineer, time and again you will come across a situation where you want to profile CPU of a system. The reasons might be many; like, CPU usage being high, you want to trace a method to see its CPU cost or you suspect CPU times for a slow transaction.
You might use one of the various profilers out there to do this. (I use yourkit and Jprofiler). All these profilers report the CPU costs in terms of CPU Time, when you profile the CPU. This time is not the equivalent of your watch time.
So in this article, let’s try and understand what is CPU time and other fundamentals w.r.t to CPU.
Clock Cycle of CPU :
The speed of a CPU is determined by its clock cycle, which is the amount of time taken for one complete oscillation of a CPU. (which is two pulses of an oscillation). In more simple terms, consider your CPU like a pendulum. It has to go through it’s 0’s and 1’s, i.e, rising edge and falling edge. The amount of time taken for this one oscillation is the Clock Cycle of a CPU. Each CPU instruction might take one or more CPU cycles to execute.
Clock speed (or Clock rate):
This is the total number of Clock cycles that a CPU can perform in one second. Each CPU executes at a particular clock rate. In fact, Clock speed is often marketed as the primary capacity feature of a processor. A 4GHz CPU can perform 4 billion Clock Cycles per second.
Some processors are able to vary their clock rates. Increase it to improve performance or decrease it to reduce power consumption.
Cycles per instruction (CPI) :
As we know, all the requests are served by CPU in the form of instruction sets. A single request can translated in to 100’s of instruction sets. Cycles spent per instruction is an important parameter which helps understand where CPU is spending its clock cycles. This metrics can also be expressed in the inverse form, i.e, Instructions per Cycle (IPC).
It is important to note that CPI value signifies the efficiency of instruction processing , but not of the instructions themselves.
CPU Time :
After knowing the above parameters, it is much easier to understand CPU time for a program now.
A single program will have a set of Instructions. (Instructions / Program)
Each instruction will consume a set of CPU cycles. ( Cycles / Instruction )
Each CPU cycle is based on the CPU’s clock speed ( secs / cycle)
Hence, the CPU time that you see in your profiler is :
CPU Time for a process = (No. of instructions executed) X (Cycles per Instruction) X Clock Cycle.
So the next time when you see “Time” being mentioned in your Profiler, remember the above calculations.
Happy tuning.
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