Intel® Advisor Help
Purpose and Usage | Run | Chart Controls | Chart Data | What Do I Do Next?
To add a Roofline chart to the Survey Report, run a Roofline analysis that helps you visualize actual performance against hardware-imposed performance ceilings, as well as determine the main limiting factor (memory bandwidth or compute capacity), thereby providing an ideal roadmap of potential optimization steps.
Use the Roofline chart to answer the following questions:
What is the maximum achievable performance with your current hardware resources?
Does your application work optimally on current hardware resources?
If not, what are the best candidates for optimization?
Is memory bandwidth or compute capacity limiting performance for each optimization candidate?
The Intel Advisor basic roofline model, the Cache-Aware Roofline Model (CARM), offers self data capability. The Intel Advisor Roofline with Callstacks feature extends the basic model with total data capability:
Self data = Memory access, FLOPs, and duration related only to the loop/function itself and excludes data originating in other loops/functions called by it
Total data = Data from the loop/function itself and its inner loops/functions
The total-data capability in the Roofline with Callstacks feature can help you:
Investigate the source of loops/functions instead of just the loops/functions themselves.
Get a more accurate view of loops/functions that behave differently when called under different circumstances.
Uncover design inefficiencies higher up the call chain that could be the root cause of poor performance by smaller loops/functions.
In the Vectorization
Workflow tab, click the
control under
Run Roofline.
Outcome: The Intel Advisor executes the target application twice to:
Measure the hardware limitations of your machine and collect loop/function timings using the Survey analysis.
Collect FLOP and integer operations data, and memory traffic data, using the Trip Counts and FLOP analysis - this collection can take three to four times longer than the Survey analysis.
After both analyses are complete, the Intel Advisor adds a Roofline chart to the Survey Report.
To implement the Roofline with Callstacks feature:
Run the Roofline analysis with the With Callstacks checkbox enabled. Upon completion, the Intel Advisor displays a Roofline chart.
Enable the With Callstacks checkbox in the Roofline chart.
There are several controls to help you show/hide the
Roofline chart:
1 |
Click to toggle between Roofline chart view and Survey Report view. |
2 |
Click to toggle to and from side-by-side Roofline chart and Survey Report view. |
3 |
Drag to adjust the dimensions of the Roofline chart and Survey Report. |
There are several controls to help you focus on the
Roofline chart data most important to you, including the following.
1 |
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2 |
Use the Cores drop-down toolbar to:
Choose the appropriate number of CPU cores to scale roof values up or down:
By default, the number of cores is set to the number of threads used by the application (even values only). You’ll see the following options if your code is running on a multisocket PC:
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3 |
Select the Memory Level(s) to show for each loop/function in the chart (CARM, L2, L3, DRAM). Selected levels are displayed as additional labeled dots on the chart. This control requires that you set the environment variable ADVIXE_EXPERIMENTAL=int_roofline. Also be sure to enable the For All Memory Levels checkbox under Run Roofline in the Vectorization Workflow tab. |
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Display Roofline chart data from other Intel Advisor results or non-archived snapshots for comparison purposes. Use the drop-down toolbar to:
Click a loop/function dot in the current result to show the relationship (arrowed lines) between it and the corresponding loop/function dots in loaded results/snapshots. |
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Add visual indicators to the Roofline chart to make the interpretation of data easier, including performance limits and whether loops/functions are memory bound, compute bound, or both. Use the drop-down toolbar to:
The preview picture is updated as you select guidance options, allowing you to see how changes will affect the Roofline chart’s appearance. Click Apply to apply your changes, or Default to return the Roofline chart to its original appearance. |
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You can save your Roofs Settings or Point Weight Representation configuration to a JSON file or load a custom configuration. |
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Zoom in and out using numerical values. |
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Hover your mouse over an item to display metrics for it. Click a loop/function dot to:
You can also click an item in the Callstack pane to flash the corresponding loop/function dot in the Roofline chart. If Roofline with Callstacks is enabled, click a loop/function dot
Right-click a loop/function dot or a blank area in the Roofline chart to perform more functions, such as:
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If Roofline with Callstacks is enabled, show/hide the Callstack pane. |
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Display the number and percentage of loops in each loop weight representation category. |
The Roofline chart plots an application's achieved performance and arithmetic intensity against the machine's maximum achievable performance:
Arithmetic intensity (x axis) - measured in number of floating-point operations (FLOPs) and/or integer operations (INTOPs) per byte, based on the loop/function algorithm, transferred between CPU/VPU and memory
Performance (y axis) - measured in billions of floating-point operations per second (GFLOPS) and/or billions of integer operations per second (GINTOPS)
In general:
The size and color of each Roofline chart dot represent relative execution time for each loop/function. Large red dots take the most time, so are the best candidates for optimization. Small green dots take less time, so may not be worth optimizing.
Roofline chart diagonal lines indicate memory bandwidth limitations preventing loops/functions from achieving better performance without some form of optimization. For example: The L1 Bandwidth roofline represents the maximum amount of work that can get done at a given arithmetic intensity if the loop always hits L1 cache. A loop does not benefit from L1 cache speed if a dataset causes it to miss L1 cache too often, and instead is subject to the limitations of the lower-speed L2 cache it is hitting. So a dot representing a loop that misses L1 cache too often but hits L2 cache is positioned somewhere below the L2 Bandwidth roofline.
Roofline chart horizontal lines indicate compute capacity limitations preventing loops/functions from achieving better performance without some form of optimization. For example: The Scalar Add Peak represents the peak number of add instructions that can be performed by the scalar loop under these circumstances. The Vector Add Peak represents the peak number of add instructions that can be performed by the vectorized loop under these circumstances. So a dot representing a loop that is not vectorized is positioned somewhere below the Scalar Add Peak roofline.
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 greater the distance between a dot and the highest achievable roofline, the more opportunity exists for performance improvement.
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.
The following Roofline chart representation shows some of the added benefits of the Roofline with Callstacks feature, including:
A navigable, color-coded Callstack pane that shows the entire call chain for the selected loop/function, but excludes its callees
Visual indicators (caller and callee arrows) that show the relationship among loops and functions
The ability to simplify dot-heavy charts by collapsing several small loops into one overall representation
Loops/functions with no self data are grayed out when expanded and in color when collapsed. Loops/functions with self data display at the coordinates, size, and color appropriate to the data when expanded, but have a gray halo of the size associated with their total time. When such loops/functions are collapsed, they change to the size and color appropriate to their total time and, if applicable, move to reflect the total performance and total arithmetic intensity.
See the Use Automated Roofline Chart to Make Optimization Decisions tutorial to learn how to:
Address memory bandwidth bottlenecks.
Address compute capacity bottlenecks.
Identify the real bottlenecks.
An Introduction to the Roofline Feature in Intel Advisor (a video that essentially covers the same content as the tutorial, but in more conceptual depth)