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OpenACC Tutorial - Profiling: Difference between revisions
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pgc++ CXXFLAGS=-fast -Minfo=all,intensity,ccff LDFLAGS=-fast main.o -o cg.x -fast | pgc++ CXXFLAGS=-fast -Minfo=all,intensity,ccff LDFLAGS=-fast main.o -o cg.x -fast | ||
}} | }} | ||
== Computational Intensity == | |||
Computational Intensity of a loop is a measure of how much work is being done compared to memory operations. | |||
'''Computation Intensity = Compute Operations / Memory Operations''' | |||
Computational Intensity of 1.0 or greater is often a clue that something might run well on a GPU. | |||
== Understanding the code == | |||
Lets look more closely at the following code: | |||
<syntaxhighlight lang="cpp" line highlight="1,5,10,12"> | |||
for(int i=0;i<num_rows;i++) { | |||
double sum=0; | |||
int row_start=row_offsets[i]; | |||
int row_end=row_offsets[i+1]; | |||
for(int j=row_start; j<row_end;j++) { | |||
unsigned int Acol=cols[j]; | |||
double Acoef=Acoefs[j]; | |||
double xcoef=xcoefs[Acol]; | |||
sum+=Acoef*xcoef; | |||
} | |||
ycoefs[i]=sum; | |||
} | |||
</syntaxhighlight> | |||
Given the code above, we search for data dependencies: | |||
* Does one loop iteration affect other loop iterations? | |||
* Do loop iterations read from and write to different places in the same array? | |||
* Is sum a data dependency? No, it’s a reduction. | |||