[petsc-users] Difference between Block Jacobi and ILU?

[Justin Chang]

Two more questions, somewhat related maybe.

Is there a practical case where one would use plain Jacobi preconditioning
over ILU?

Also, what exactly is happening when one uses -pc_bjacobi_blocks 2 ?

Thanks,
Justin

On Wed, Jan 13, 2016 at 9:37 PM, Justin Chang <jychang48 at gmail.com> wrote:

> Thanks Satish,
>
> And yes I meant sequentially.
>
> On Wed, Jan 13, 2016 at 8:26 PM, Satish Balay <balay at mcs.anl.gov> wrote:
>
>> On Wed, 13 Jan 2016, Justin Chang wrote:
>>
>> > Hi all,
>> >
>> > What exactly is the difference between these two preconditioners? When I
>> > use them to solve a Galerkin finite element poisson problem, I get the
>> > exact same performance (iterations, wall-clock time, etc).
>>
>> you mean - when you run sequentially?
>>
>> With block jacobi - you decide the number of blocks. The default is
>> 1-block/proc
>> i.e - for sequnetial run you have only 1block i.e  the whole matrix.
>>
>> So the following are essentially the same:
>> -pc_type bjacobi -pc_bjacobi_blocks 1 [default] -sub_pc_type ilu [default]
>> -pc_type ilu
>>
>> Satish
>>
>> > Only thing is I can't seem to use ILU in parallel though.
>>
>>
>
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