This message is a bit cryptic. It’s coming from this section in leqsol_mod.f
CALL DGTSV(NEND-NSTART+1, 1, CC(NSTART+1), DD, EE, BB, NEND-NSTART+1, INFO
)
IF (INFO.NE.0) THEN
write(*,*) 'leq_iksweep',INFO, myPE
IF(DMP_LOG)WRITE (UNIT_LOG,*) 'ROUTINE = ', ' IKSWEEP'
RETURN
ENDIF
DGTSV is a library routine that solves a tridiagonal matrix equation AX=Y for X. The nonzero return code for INFO means that A was a degenerate matrix.
In the current 21.2 version, we replaced DGTSV with a more efficient algorithm that runs up to 2x faster (modified Thomas algorithm). However with the new code, instead of this error message, we get a zero-division and the solver exits. We probably need to do something more to address this degenerate case.
– Charles