Allow Multiple Runs or Multiplex Events
Enable multiple runs of the event-based sampling data collection for more accurate analysis results.
Intel® VTune™ Profiler runs the hardware event-based sampling analysis to collect data based on the events defined for the selected analysis type. The number of events it can monitor during a single run is limited by the number of performance counters in your processor. If you enable multiple runs of the data collection, the VTune Profiler runs the hardware event-based sampling data collector as many times as required to collect data on all the events specified for the analysis type. If you specified an application to launch as an analysis target, the VTune Profiler launches your application each time the hardware event-based sampling collector runs.
VTune Profiler allows to avoid multiple runs of the data collection by multiplexing the use of physical counters within a single sampling run. Event multiplexing removes the need for multiple runs of the application, thereby reducing the time needed to complete sampling collection at the cost of lower precision of the result data. Event sample counts collected in the multiplexed mode are extrapolated to the total collection runtime.
Event multiplexing is also useful if the application does not have a long steady state or takes a long time to get to steady state. On the other hand, if application initialization is short and it gets to steady state quickly, then you can do multiple short runs and will not need to do event multiplexing.
To enable/disable multiple runs of the data collection:
- Click the Configure Analysis button on the VTune Profiler toolbar.
The Configure Analysis window opens.
- Specify your target system type and select the Application to Launch target type.
Collecting data in multiple runs is only possible if an application to launch is specified.
- On the WHAT configuration pane, scroll down to the Advanced section and select the Allow multiple runs option to enable more precise event data collection or deselect the option to use event multiplexing.
If you enable the multiple run mode, the VTune Profiler runs the data collection several times for each event set. You can easily detect these multiple runs on the Timeline pane: they are separated with the grayed out paused areas.
The multiple run mode affects the metrics calculation. All "total" types of metrics (Total Time, Elapsed Time) are calculated for the whole analysis session that includes multiple runs while all other metrics are provided per run.
If you want to avoid running the application multiple times but get more accurate multiplexing data, you need to create a custom analysis and enable the Use precise multiplexing option available for the custom hardware event-based sampling analysis configuration. This option enables a multiplexing algorithm that switches event groups on each sample. This mode provides more reliable statistics for applications with a short execution time. You may also consider enabling the precise multiplexing if the MUX Reliability metric for the Microarchitecture Exploration analysis result is low.