4.42.3.1 Profiling in the Windows Implementation

Profiling in the Windows version is similar, but as profiling is a statistical process it is good to be aware of the implementation^{166We hereby acknowledge Lionel Fourquaux, who suggested the design described here after a newsnet enquiry.} for proper interpretation of the results.

Windows does not provide timers that fire asynchronously, frequent and proportional to the CPU time used by the process. Windows does provide multi-media timers that can run at high frequency. Such timers, however, run in a separate thread of execution and they are fired on the wall clock rather than the amount of CPU time used. The profiler installs such a timer running, for saving CPU time, rather inaccurately at about 100 Hz. Each time it is fired, it determines the CPU time in milliseconds used by Prolog since the last time it was fired. If this value is non-zero, active predicates are incremented with this value.