A target is an executable file the

Intel Advisor

can analyze.

To build applications that produce the most accurate and complete

Vectorization Advisor

analysis results, build an optimized binary of your application in release mode using the following settings:

/ZI
/DEBUG
/Qopt-report:5
/O2
or higher
/Qvec
/Qsimd
/Qopenmp

The Roofline analysis is a combination of the Survey analysis followed immediately by the Trip Counts/FLOPs analysis. The Trip Counts/FLOPs analysis may run three to four times longer than the Survey analysis.

The size and color of each

Roofline

chart dot represent relative execution time for each loop/function. Large red dots take the most time; small green dots take less time.

Horizontal

Roofline

chart lines (

rooflines

) indicate compute capacity limitations preventing loops/functions from achieving better performance without some form of optimization.

Diagonal

Roofline

chart lines indicate memory bandwidth limitations preventing loops/functions from achieving better performance without some form of optimization.

A dot cannot exceed the topmost rooflines, as these represent the maximum capabilities of the machine; however, not all loops can utilize maximum machine capabilities.

The best candidates for the greatest performance improvement are large, red dots that are farther from the topmost achievable roofline.

The

Roofline

chart offers a variety of controls to configure appearance and focus on data of interest.

Memory bandwidth bottlenecks are generally overcome with cache optimizations.

Check data in other

Intel Advisor

views to support your

Roofline

chart interpretation.

Arithmetic Intensity (the x-axis of the

Roofline

chart) = Floating-point operations per byte accessed. Any given algorithm has an arithmetic Intensity. In theory, optimization should not change this metric because it is a trait of the algorithm itself. So dots on a

Roofline

chart move up and down as performance changes, but rarely side to side.

Optimizing a loop is not enough to make the corresponding dot rise to the next roofline; a loop must make

good

use of the optimization. Inefficient vectorization is not good enough; an isolated fused multiply-add instruction (FMA) is not good enough.

In the right circumstances, you can use data layout and memory access optimizations to overcome both compute capacity and memory bandwidth limitations.

Take advantage of code-specific

how-can-I-fix-this-issue?

advice in the

Recommendations

tab.

The first roofline above a dot position isn't always the bottleneck; any roofline above a dot position could be the culprit

Even a roofline below a dot position can be a bottleneck; however, the farther a dot is positioned above a roofline, the less likely that roofline is causing the bottleneck.

If the first roofline above a dot position does not make logical sense, investigate the next roofline, and just keep working your way up the

Roofline

chart, using common sense, other

Intel Advisor

features, and your familiarity with your application to inform your investigation.

The

Roofline

chart is not a Data-In-Answers-Out utility; however, it puts you in the ballpark and guides you in the right direction to optimize your code.