# 10.7 Calibration

The profiler class has a hard coded constant that is added to each event handling time to compensate for the overhead of calling the time function, and socking away the results. The following procedure can be used to obtain this constant for a given platform (see discussion in section Limitations above).

    import profile
    pr = profile.Profile()
    pr.calibrate(100)
    pr.calibrate(100)
    pr.calibrate(100)

The argument to calibrate() is the number of times to try to do the sample calls to get the CPU times. If your computer is very fast, you might have to do:

    pr.calibrate(1000)

or even:

    pr.calibrate(10000)

The object of this exercise is to get a fairly consistent result. When you have a consistent answer, you are ready to use that number in the source code. For a Sun Sparcstation 1000 running Solaris 2.3, the magical number is about .00053. If you have a choice, you are better off with a smaller constant, and your results will ``less often'' show up as negative in profile statistics.

The following shows how the trace_dispatch() method in the Profile class should be modified to install the calibration constant on a Sun Sparcstation 1000:

    def trace_dispatch(self, frame, event, arg):
        t = self.timer()
        t = t[0] + t[1] - self.t - .00053 # Calibration constant

        if self.dispatch[event](frame,t):
            t = self.timer()
            self.t = t[0] + t[1]
        else:
            r = self.timer()
            self.t = r[0] + r[1] - t # put back unrecorded delta
        return

Note that if there is no calibration constant, then the line containing the callibration constant should simply say:

    t = t[0] + t[1] - self.t  # no calibration constant

You can also achieve the same results using a derived class (and the profiler will actually run equally fast!!), but the above method is the simplest to use. I could have made the profiler ``self calibrating'', but it would have made the initialization of the profiler class slower, and would have required some very fancy coding, or else the use of a variable where the constant ``.00053`' was placed in the code shown. This is a VERY critical performance section, and there is no reason to use a variable lookup at this point, when a constant can be used.