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 those time variables are defined by your OS, you can retrieve information on how they are calculated by executing man time in your shell (on Unix):

...These statistics consist of (i) the elapsed real time between invocation and termination, (ii) the user CPU time (the sum of the tms_utime and tms_cutime values in a struct tms as returned by times(2)), and (iii) the system CPU time (the sum of the tms_stime and tms_cstime values in a struct tms as returned by times(2)).

The definition of the mentioned time variables can be found here:

tms_utime User CPU time.

tms_stime System CPU time.

tms_cutime User CPU time of terminated child processes.

tms_cstime System CPU time of terminated child processes.

A a clarification of the differences between user and system time is described in daroczig's answer and elsewhere on SO:

The tms_utime element is the amount of time spent executing your code, or the code in the C library. The tms_stime element is the amount of time spent in the kernel executing code on your behalf.