[petsc-users] How to turn off preconditioner in PETSC?

[我]

Hi,
I want to compare the time cost between preconditioner and unpreconditioner in 
PETSc. But I didn't know how to turn off the preconditioner in Petsc. If I 
choose the PCNONE, but the solution even can not converge. If I do not declare 
PC at the beginning of my program, will PETSc choose a default preconditioner? 
I just want to turn off it. Any suggestions?
Thank you very much!
Daye