How the DLL Interfaces with a Performance Monitor Application

An application retrieves performance data by specifying HKEY_PERFORMANCE_DATA in a call to the RegQueryValueEx1J.0ZZ. function. If successful, RegQueryValueEx fills a buffer of the application with the requested performance data.

The first time an application calls RegQueryValueEx, or if the application uses the RegOpenKeyWG.8D function to open HKEY_PERFORMANCE_DATA, the system calls the Open function for each application with the correct registry entries. This gives each performance DLL an opportunity to initialize its performance data structures. Then, if the Open function returns successfully (or if there is no Open function), the system calls the Collect function. Subsequent calls to RegQueryValueEx cause the system to call the Collect function.

When the application has finished collecting performance data, it specifies HKEY_PERFORMANCE_DATA in a call to the RegCloseKey0F79.0 function. This causes the system to call the Close function for each application. The performance DLLs are then unloaded.

Note  It is possible for multiple programs to collect performance data at the same time. The system calls Open and Close functions only once for each application requesting performance data. For remote measurement, the system limits access to these routines to one thread at a time, so synchronization is not a problem. However, for local measurement, because multiple processes may be making simultaneous calls, you must prevent any conflicts from multiple concurrent requests for data.