What are 'user' and 'system' times measuring in R system.time(exp) output?

Question

I am using system.time(expression) to measure execution time for an R function.

The output I get for the call

system.time(myfunction())
         


        

Answer 1

Since these are generic anyway, from Wikipedia:

The term 'user CPU time' can be a bit misleading at first. To be clear, the total time (real CPU time) is the combination of the amount of time the CPU spends performing some action for a program and the amount of time the CPU spends performing system calls for the kernel on the program's behalf. When a program loops through an array, it is accumulating user CPU time. Conversely, when a program executes a system call such as exec or fork, it is accumulating system CPU time.

http://en.wikipedia.org/wiki/Time_(Unix)#User_Time_vs_System_Time