Confusion on Summary of Ellapsed Time

[KK1]

When I profile my code, the summary had shown the elapsed time value (1283.389) and the subset of CPU Time value is (3685.21).  So, the CPU time should be smaller than the elapsed time.  If not, what is the cpu time included?  

[Peter_W_Intel]

CPU time = summary of CPU time on cores/threads.

Please consider parallelism when profiling multithreaded application. 

[KK1]

If I run the code without VTune from the terminal, it takes only ~18 minutes.  This timing is closed to "elapsed time", but it is about ~3 minutes extra, which executing in VTune environment.  I'm still not sure what is "CPU time", the time specifies much more (61 minutes).  

It is a big different comparing 18 minutes vs 61 minutes.

[Peter_W_Intel · Accepted solution]

Elapsed time is approximated to execution time, a little overhead from VTune.

If your program is serial, Elapsed time = CPU time (single thread).

As I said previous post, if your program is multiple threads, CPU time includes all time spent on different threads (cores) - they are parallel. Is it hard to understand? For example :

                T1        T2        T3        T4

Thread1   1          1          1           1 

Thread2               1          1

Thread3                           1           1

Elapsed time = 4

CPU time = 4+2+2 = 8

[KK1]

Great. It is clear now. Thank you.