Interpreting Hotspots Data

Identify program units that took the most CPU time (hotspots) by using the Intel® VTune™ Amplifier's Hotspots viewpoint. This viewpoint is available for all analysis results.

To interpret the performance data provided in the Hotspots viewpoint, you may follow the steps below:

  1. Define a performance baseline.

  2. Identify the hottest function.

  3. Identify algorithm issues.

  4. Analyze source.

  5. Explore other analysis types.

Define a Performance Baseline

Start with exploring the Summary window that provides general information on your application execution. Note that the Elapsed time, which includes application time from start to termination, differs from the application CPU time, which is a sum of the active (does not include waiting time) processor time for all the threads that run the application.

Use the Elapsed time value provided in the Summary window as a baseline for comparison of versions before and after optimization. Note that while tuning the application, the Elapsed time tends to decrease whereas the CPU time may increase with adding more threads to the application.

Identify the Hottest Function

The basic performance data is provided in the Bottom-up and Top-down Tree panes. Use this data to identify the hottest functions in your application.

By default, the data in the Bottom-up pane is sorted in the descending order providing the most time-consuming functions first. Focus on the functions with the largest CPU time. These are your candidates for optimization.

To get more details on how effectively the CPU time was used, switch to the Hotspots by CPU Usage viewpoint for the results of the user-mode sampling and tracing collection and expand the CPU Time column:

Hotspots by CPU Usage Viewpoint: Bottom-up Pane

For the hardware event-based sampling results, you do not need to switch to a different viewpoint. CPU utilization data is available when you expand the Effective Time by Utilization column by clicking the symbol.

Focus your tuning efforts on the program units with the largest Poor value. This means that during the execution of these program units your application underutilized the CPU time. The overall goal of optimization is to achieve Ideal (green ) or OK (orange ) CPU usage state and shorten the Poor and Over CPU usage values.

Identify Algorithm Issues

You can identify issues with the calling sequences in your application and improve performance by revising the way functions are called. The following methods to locate potential issues are available:

  • Top-down Tree pane: Analyze the Total and Self time data for callers and callees of the hotspot function to understand whether this time can be optimized.

  • Call Stack pane: Identify the highest contributing stack for the program unit(s) selected in the Bottom-up or Top-down Tree panes. Use the navigation buttons to see the different stacks that called the selected program unit(s). The contribution bar shows the contribution of the currently visible stack to the overall time spent by the selected program unit(s). You can also use the drop-down list in the Call Stack pane to view data for different types of stacks.

Analyze Source

Double-click the hottest function to view its related source code file in the Source/Assembly window. You can open the code editor directly from the Intel® VTune™ Amplifier and edit your code (for example, minimizing the number of calls to the hotspot function).

Explore Other Analysis Types

For more complete information about compiler optimizations, see our Optimization Notice.