Comparisons of Timing Metrics

If you compare timing metrics obtained from the profiling done in a clock-based experiment with times obtained by other means, you should be aware of the following issues.

For a single-threaded application, the total Solaris LWP or Linux thread time recorded for a process is usually accurate to a few tenths of a percent, compared with the values returned by gethrtime(3C) for the same process. The CPU time can vary by several percentage points from the values returned by gethrvtime(3C) for the same process. Under heavy load, the variation might be even more pronounced. However, the CPU time differences do not represent a systematic distortion, and the relative times reported for different functions, source-lines, and such are not substantially distorted.

For multithreaded applications using unbound threads on the Solaris OS, differences in values returned by gethrvtime() could be meaningless because gethrvtime() returns values for an LWP, and a thread can change from one LWP to another.

The LWP times that are reported in the Performance Analyzer can differ substantially from the times that are reported by vmstat, because vmstat reports times that are summed over CPUs. If the target process has more LWPs than the system on which it is running has CPUs, the Performance Analyzer shows more wait time than vmstat reports.

The microstate timings that appear in the Statistics tab of the Performance Analyzer and the er_print statistics display are based on process file system /proc usage reports, for which the times spent in the microstates are recorded to high accuracy. See the proc (4) man page for more information. You can compare these timings with the metrics for the <Total> function, which represents the program as a whole, to gain an indication of the accuracy of the aggregated timing metrics. However, the values displayed in the Statistics tab can include other contributions that are not included in the timing metric values for <Total>. These contributions come from the periods of time in which data collection is paused.

User CPU time and hardware counter cycle time differ because the hardware counters are turned off when the CPU mode has been switched to system mode. For more information, see Traps.