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Profiler: VTune

What is VTune?

VTune is a tool that allows you to quickly identify where most of the execution time of a program is spent. This is known as profiling. It is good practice to profile a code before attempting to modify the code to improve its performance. VTune collects key profiling data and presents them in an intuitive way.

Where to find more resources on VTune?

How to use VTune?

VTune is available on Mahuika by loading the VTune module.

module spider VTune

First login to Mahuika and then run:

module load VTune

We'll show how to profile a C++ code with VTune - feel free to choose your own code instead. Start with 

git clone https://github.com/pletzer/fidibench

and build the code using the "gimkl" tool chain

module load gimkl CMake
cd fidibench
mkdir build
cd build
cmake .. -D CMAKE_BUILD_TYPE=RelWithDebInfo
make

This will compile a number of executables. Note that VTune does not require one to apply a special compiler switch to profile.

We choose "upwindCxx" as the executable to profile. It is under upwind/cxx, so 

cd upwind/cxx

Run the executable with 

rm -rf vtune-res*
srun --mem=20g --ntasks=32 vtune -collect hotspots -result-dir vtune-res ./upwindMpiCxx -numCells 512 -numSteps 10
This will print out 
Top Hotspots
Function                    Module          CPU Time  % of CPU Time(%)
--------------------------  --------------  --------  ----------------
MultiArrayIter::getIndices  upwindMpiCxx     78.197s             21.4%
Filter::applyFilter         upwindMpiCxx     55.929s             15.3%
operator new                libstdc++.so.6   28.083s              7.7%
_int_free                   libc.so.6        26.226s              7.2%
getdelim                    libc.so.6        18.724s              5.1%
[Others]                    N/A             158.460s             43.3%
which lists the functions and methods accounting for most of the execution time. A "vtune-res*" directory storing the profiling data will also be created. You can also get a graphical view of the profiling data with

vtune-gui vtune-res.ml16.hpc.nesi.org.nz &
(where vtune-res.ml16.hpc.nesi.org.nz should be replaced by your results directory).

An example is the flame graph VTune_FlameGraph.png