Set the overhead for tracing system calls
-O option is used to set the overhead for tracing system calls, and the unit is microseconds (us). The following is excerpted from manual:
This is useful for overriding the default heuristic for guessing how much time is spent in mere measuring when timing system calls using the -c option. The accuracy of the heuristic can be gauged by timing a given program run without tracing (using time(1)) and comparing the accumulated system call time to the total produced using -c.
Check the source code:
...... ts_mul(&dtv, &overhead, counts[i].calls); ts_sub(&counts[i].time, &counts[i].time, &dtv); ......
The overhead time is subtracted from the total consuming time, and this option just makes sure the measure of time spent on system calls to be more accurate.