Intel® Advisor User Guide

User Guide

Contents

Examine Bottlenecks on CPU Roofline Chart

Accuracy Level

Low

Enabled Analyses

Survey + FLOP (Characterization)

Result Interpretation

Example of a Cache-aware CPU Roofline chart
The farther a dot is from the topmost roofs, the more room for improvement there is. In accordance with Amdahl's Law, optimizing the loops that take the largest portion of the program's total run time will lead to greater speedups than optimizing the loops that take a smaller portion of the run time.
  • By dot size and color, identify loops that take most of total program time and/or located very low in the chart. For example:
    • Small, green dots take up relatively little time, so are likely not worth optimizing.
    • Large, red dots take up the most time, so the best candidates for optimization are the large, red dots with a large amount of space between them and the topmost roofs.
    You can switch between coloring the dots by execution time and coloring the dots by type (scalar or vectorized) in the roof view menu on the right.
  • Depending on the dots position, identify what the loops are bounded by.
    Intel® Advisor
     marks the roofline zones on the chart to help you identify what roofs bound the loop:
    • Loop is bounded by memory roofs.
    • Loop is bounded by compute roofs.
    • Loop is bounded by both memory and compute roofs.
  • Select a dot on the chart, open the
    Code Analytics
     tab, and refer to the Roofline pane for more details about a specific roof that bounds the loop.
  • In the
    Recommendations
     tab, scroll down to the
    Roofline Guidance
     section that provides you hints on next optimization steps for a selected loop/function.
The roofs above a dot represent the restrictions preventing it from achieving a higher performance, although the roofs below can contribute somewhat. Each roof represents the maximum performance achievable without taking advantage of a particular optimization, which is associated with the next roof up. Depending on a dot position, you can try the following optimizations.
For more precise optimization recommendations, see the Roofline Guidance in
Code Analytics
 and Roofline Conclusions in
Recommendations
 tabs.
Dot Position
Reason
To Optimize
Below a memory roof (DRAM Bandwidth, L1 Bandwidth, so on)
The loop/function uses memory inefficiently.
Run a Memory Access Patterns analysis for this loop.
  • If MAP analysis suggests cache optimization, make any appropriate optimizations.
  • If cache optimization is impossible, try reworking the algorithm to have a higher AI.
Below Vector Add Peak
The loop/function under-utilizes available instruction sets.
Check
Traits
 column in the Survey report to see if FMAs are used.
  • If FMA is not used, try altering your code or compiler flags to induce FMA usage.
Just above Scalar Add Peak
The loop/function is undervectorized.
Check vectorization efficiency and performance issues in the Survey. Follow the recommendations to improve it if it's low.
Below Scalar Add Peak
The loop/function is scalar.
Check the
Survey
 report to see if the loop vectorized. If not, try to get it to vectorize if possible. This may involve running Dependencies to see if it's safe to force it.
In the following
Roofline
 chart representation, loops A and G (large red dots), and to a lesser extent B (yellow dot far below the roofs), are the best candidates for optimization. Loops C, D, and E (small green dots) and H (yellow dot) are poor candidates because they do not have much room to improve or are too small to have significant impact on performance.
This is a visual model, not an actual screenshot, of the Roofline Chart
Some algorithms are incapable of breaking certain roofs. For instance, if Loop A in the example above cannot be vectorized due to dependencies, it cannot break the Scalar Add Peak.
If you cannot break a memory roof, try to rework your algorithm for higher arithmetic intensity. This will move you to the right and give you more room to increase performance before hitting the memory bandwidth roof. This would be the appropriate approach to optimizing loop F in the example, as well as loop G if its cache usage cannot be improved.

Product and Performance Information

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Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.