About Real time, User time and SYS time

In OS CPU Real, User and Sys process time statistics

One of these things is not like the other. Real refers to actual elapsed time; User and Sys refer to CPU time used only by the process.

1) What is Real Time

Real time is wall clock time - It's the time taken to start and finish the entire execution. 

2) What is User Time

It is the  the amount of CPU time spent in user-mode code (outside the kernel) within the process. This is only actual CPU time used in executing the process. Other processes and time the process spends blocked do not count towards this figure. If you want to know more about CPU time there is separate post where CPU time is well explained.

2) What is System Time

System time is the amount of CPU time spent in the kernel within the process. Which means executing CPU time spent in system calls within the kernel, as opposed to library code, which is still running in user-space. Like 'user', this is only CPU time used by the process. See below for a brief description of kernel mode (also known as 'supervisor' mode) and the system call mechanism.

User+Sys -  will tell you how much actual CPU time your process used. Note that this is across all CPUs, so if the process has multiple threads (and this process is running on a computer with more than one processor) it could potentially exceed the wall clock time reported by Real (which usually occurs). 

The rule of thumb is:

real < user: The process is CPU bound and takes advantage of parallel execution on multiple cores/CPUs.

real ≈ user: The process is CPU bound and takes no advantage of parallel exeuction.

real > user: The process is I/O bound. Execution on multiple cores would be of little to advantage

http://stackoverflow.com/questions/556405/what-do-real-user-and-sys-mean-in-the-output-of-time

A program is CPU bound if it would go faster if the CPU were faster, i.e. it spends the majority of its time simply using the CPU (doing calculations). A program that computes new digits of π will typically be CPU-bound, it's just crunching numbers.

A program is I/O bound if it would go faster if the I/O subsystem was faster. Which exact I/O system is meant can vary; I typically associate it with disk. A program that looks through a huge file for some data will often be I/O bound, since the bottleneck is then the reading of the data from disk.

CPU Bound means the rate at which process progresses is limited by the speed of the CPU. A task that performs calculations on a small set of numbers, for example multiplying small matrices, is likely to be CPU bound.

I/O Bound means the rate at which a process progresses is limited by the speed of the I/O subsystem. A task that processes data from disk, for example, counting the number of lines in a file is likely to be I/O bound.

Memory bound means the rate at which a process progresses is limited by the amount memory available and the speed of that memory access. A task that processes large amounts of in memory data, for example multiplying large matrices, is likely to be Memory Bound.

Cache bound means the rate at which a process progress is limited by the amount and speed of the cache available. A task that simply processes more data than fits in the cache will be cache bound.

I/O Bound would be slower than Memory Bound would be slower than Cache Bound would be slower than CPU Bound.