For many applications such as nonlinear problems and the time-accurate solution of time dependent PDE's, it is necessary to solve a sequence of systems for which the matrix does not change or changes to a slight degree in the values of its entries between solves. In such cases it is desirable to reuse the setups for the matrix vector product operation and the preconditioner across the solves.
The top-level PCG argument IJOB is used to control solution of multiple systems. Typically, IJOB is set to JIRT, to indicate that PCG should initialize, run the iterative method, and terminate. However, calls with other settings such as JINIT, JRUN and JTERM may be used to execute repeated solves. The IPARM variable IPTR is used to reference the result of a setup call.