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()) 

is:

    user  system elapsed      117.36    5.65  127.86 

What does 'user' and 'system' measure?

Answer 1

This is discussed in ?proc.time (system.time() returns an object of class "proc.time"):

Details:       ‘proc.time’ returns five elements for backwards compatibility, but      its ‘print’ method prints a named vector of length 3.  The first      two entries are the total user and system CPU times of the current      R process and any child processes on which it has waited, and the      third entry is the ‘real’ elapsed time since the process was      started. 

....and

Value:  ....       The definition of ‘user’ and ‘system’ times is from your OS.      Typically it is something like       _The ‘user time’ is the CPU time charged for the execution of user      instructions of the calling process. The ‘system time’ is the CPU      time charged for execution by the system on behalf of the calling      process._ 

Answer 2

The clearest explanation I've ever read on the difference between user and system elapsed time was provided by William Dunlap on [R-help]:

"User CPU time" gives the CPU time spent by the current process (i.e., the current R session) and "system CPU time" gives the CPU time spent by the kernel (the operating system) on behalf of the current process. The operating system is used for things like opening files, doing input or output, starting other processes, and looking at the system clock: operations that involve resources that many processes must share.

Although ?proc.time returns something similar, this description was a lot easier to understand for me.