# [Trilinos-Users] [EXTERNAL] Question about GCRODR in Belos

Wen Yan wenyan4work at gmail.com
Fri Aug 10 11:28:17 EDT 2018

Thank you for the explanation. I will look into that.

Best,

Wen

On Thu, Aug 9, 2018 at 11:28 PM Parks, Michael L <mlparks at sandia.gov> wrote:

> Wen,
>
>
>
>    You can find a better description of the method, including pseudocode
> for the solver, in the associated paper *Recycling Krylov Subspaces for
> Sequences of Linear Systems* at
> https://epubs.siam.org/doi/abs/10.1137/040607277.
>
>
>
>    The application of the operator to the recycle space is in general
> necessary to update the image of that space if the operator has changed
> from one linear system to the next. (See instructions 2-19 in the
> pseudocode in the paper.) If the operator has not changed, as in your case,
> much of this work can be skipped. The GCRODR implementation in Belos
> assumes the operator may change from one solve to the next, so you would
> have to modify the solver to avoid this extra work if this is not the case.
>
>
>
> Best,
>
>
>
> – Mike
>
>
>
> ---
>
> Michael L. Parks, Ph.D.
>
> Manager, Computational Mathematics Dept.
>
> Center for Computing Research
>
> Sandia National Laboratories
>
> www.sandia.gov/~mlparks
>
>
>
> -----Original Message-----
> From: Trilinos-Users [mailto:trilinos-users-bounces at trilinos.org] On
> Behalf Of Wen Yan
> Sent: Thursday, August 9, 2018 10:43 AM
> To: trilinos-users at trilinos.org
> Subject: [EXTERNAL] [Trilinos-Users] Question about GCRODR in Belos
>
>
>
> Dear Trilinos users,
>
>
>
> I am trying to use GCRODR to solve a sequence of linear systems Ax_i=b_i,
> where the operator A does not change but the right side b_i slightly
> changes. The setup of GCRODR works, and the reported number of iterations
> after the first linear system is reduced compared to cases using
> PseudoBlockGMRES. However, I found that inside GCRODR->solve() and before
> the iterations, the operator A is applied to one multi vector with column
> number equal to the number of recycled vectors. Therefore there is no
> saving in computational time. I was using default parameters for GCRODR.
>
>
>
> I was wondering if this is the expected behavior of GCRODR, or if there is
> a switch controlling it. I didn’t find that on the document webpage.
>
>
>
> Thank you!
>
> Wen Yan
>
>
>
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>
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>
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>
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