[petsc-users] fieldsplit error

[Jed Brown]

Hong Zhang <hzhang at mcs.anl.gov> writes:

> subramanya:
>
>>  Hi,
>> I figured out that block size was missing. However I am not quite sure
>> what the block size means.
>>
>
> number of fields.
> See
>  ./ex19 -snes_monitor_short -ksp_monitor_short -pc_type fieldsplit
> -pc_fieldsplit_block_size 2 -pc_fieldsplit_0_fields 0,1
> -pc_fieldsplit_1_fields 0,1 -pc_fieldsplit_type multiplicative -snes_view
> -fieldsplit_pc_type lu -da_refine 1 -ksp_type fgmres -h |grep
> fieldsplit_block_size

Hong is correct, but this is a very confusing command-line to use as an
explanation.  If you read ex19, it defines four fields.  The option is
forcing PCFIELDSPLIT to interpret the array as having only two fields.
Then both fields are being put into both splits, with a direct solver on
both splits.  I.e., the entire problem is being solved with a direct
solver in both stages of a two-stage multiplicative preconditioner.
This is a test to demonstrate that a weird code path works, but it's
nothing you would actually use in practice.
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