When is garbage collection initiated?

I need a better understanding of when garbage collection is initiated under PHP's default session handling. It is not clear to me if the gc_probability is based on: - each *request* to the server, or, - each time a *new session file is created*, or, - each time a request causes a *session file to be created or accessed*. Does anyone know this? Below is the documentation I found. I added the underscores for emphasis. - At http://www.php.net/manual/en/ref.session.php#AEN83911: "session.gc_probability specifies the probability that the gc (garbage collection) routine is started on each _request_ in percent." - In the php.ini file: "; Percentual probability that the 'garbage collection' process is started ; on every _session initialization_." TIA Kirk