Library /sys$common/syshlp/pca$help.hlb SET, VPC_SAMPLING, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The SET VPC_SAMPLING Collector command enables the collection of
    PC values for random vector instructions and causes the sampling
    rate to be applied to the wall clock. The collected data lets you
    determine the scalar/vector parallelism throughout your entire
    program.

    When you collect vector PC samples, the sampling interval timer
    includes all idle time associated with the current run of the
    program. This form of sampling shows you where the time is being
    spent in the program with little cost to the time of actually
    running the program. time. You can set the time interval by using
    the /INTERVAL qualifier.