CodeXL User GuideWhen time-based profiling is enabled and started, CodeXL configures a timer that periodically interrupts the program executing on a processor core.
When a timer interrupt occurs, a sample is created and saved for post-processing. Post-processing builds up a type of histogram, which describes what the system and its software components were doing. The most time-consuming parts of a program have the most samples because, most likely, the program is executing in those regions when a timer interrupt is generated and a sample is taken.
The frequency of sample taking is controlled by the timer interval. This sometimes is called the "sampling period." The timer interval is 1 millisecond: roughly 1,000 TBP samples are taken each second for each processor core.
The second factor is the length of time during which the samples are taken.. The measurement period depends on the overall execution time of the workload and the way in which CodeXL data collection is configured. Using the CPU Profile Options, CodeXL can be configured to collect samples for all, or part, of the time that the test workload is executing. If program run-time is short (less than 15 seconds), it may be necessary to increase program run-time by using a larger data set or more loop iterations to obtain a statistically useful result.
Deciding how many samples are enough requires a working knowledge about the characteristics of the workload under test. Scientific applications often have tight inner loops that are executed several times. In these situations, samples are being aggregated rapidly within the inner loops, and even a fairly short run-time yields a statistically useful number of samples. Other workloads, like transaction processing, usually have just a few inner loops, and the profiles are relatively "flat." For flat workloads, a longer measurement period is required to aggregate samples in code regions of interest.