Effective solution of linear systems often requires careful selection of methods as well as various iteration parameters. The PCG arrays IPARM and FPARM are supplied to communicate these options to the package, and in turn to supply the user with feedback on the iteration process. Some of the more commonly used parameters include the convergence tolerance ZETA, the maximum allowable number of iterations ITSMAX, the desired level of printed output to the user LEVOUT, and the size of the system on the given processor NRU.
The routine _DFALT is supplied to set the parameter arrays to the most commonly used default settings. Its usage is
CALL _DFALT (IPARM,FPARM)
After making this call, the user can modify appropriate entries of IPARM and FPARM as desired. The Fortran include file pcg_fort.h is supplied which defines mnemonics for entries into these arrays to simplify setting or reading those values.