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linux time command vs vtune's elapsed time

New Contributor I

Hi all,
I am trying to analyze an application via vtune amplifier (parallel studio 2016). I had used following command line:
time mpirun -np 1 amplxe-cl -collect advanced-hotspots -result-dir result_$HOSTNAME ./myApp
and i got:-
real    96m46.763s
user    365m52.038s
sys    7m6.127s

And vtune's GUI reports:-

Elapsed Time:    345.404s
    CPU Time:    4287.747s
        Effective Time:    1095.314s
            Idle:    0.176s
            Poor:    407.717s
            Ok:    47.757s
            Ideal:    639.664s
            Over:    0s
        Spin Time:    3191.179s
            Imbalance or Serial Spinning (OpenMP):    229.153s
            Lock Contention (OpenMP):    2955.375s
            Communication (MPI):    0s
            Other:    6.651s
        Overhead Time:    1.254s
            Creation (OpenMP):    0s
            Scheduling (OpenMP):    0s
            Reduction (OpenMP):    0s
            Other:    1.254s
    Instructions Retired:    5,937,272,500,000
    CPI Rate:    1.803
    CPU Frequency Ratio:    1.000
    Paused Time:    0s

Earlier i had an idea that that Vtune's reported "elapsed time" is the actual elapsed time of application.
Now i am bit confused .

Could anyone please throw a light on this issue.
Eagerly awaiting your replies.


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3 Replies
Honored Contributor III

The Linux "time" command will include the execution time of your job, plus execution time of the VTune post-processing step(s).   These post-processing steps can be very time-consuming....

It is also possible that the "Elapsed Time" that VTune reports is an estimate based on the actual elapsed time minus an estimate of the overhead introduced by VTune.  

If you re-run your code without VTune, the elapsed time should help make it clear how to interpret the VTune output.

Varying the number of OpenMP threads and seeing how this changes the VTune time breakdown should also help....

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New Contributor I

After recompilation (without -g flag) and re-run , instance #1 took:-
real    92m47.692s
user    365m15.774s
sys    6m44.852s

instance #2:-
real    89m16.851s
user    368m14.433s
sys    5m53.945s

It seems:  profiled - non profiled  =~ 5 minutes.
Does the elapsed time(Elapsed Time:    345.404s or ~5 Minutes) in the vtune log means - the time taken by vtune for collecting data  ??

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John is right - Linux time command in your case will measure application under vtune collection time (that is Elapsed time that VTune shows) + Vtune result finalization time that can be significant.

So if you want to compare what Linux time command shows towards VTune elapsed time you can do the following:

mpirun -np 1 amplxe-cl -collect advanced-hotspots -result-dir result_$HOSTNAME time ./myApp

To avoid confusions - application elapsed time that VTune shows is elapsed time of the application under profiling and it will include the overhead that the tool introduces if any.

Also I would recommend to upgrade VTune to 2017 Update 1 and try the new HPC Performance Characterization analysis "hpc-performance" command line name - it will show you also OpenMP efficiency metrics along with memory/FPU metrics in a unified report.

Thanks & Regards, Dmitry



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