[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Question regarding PCMG
- To: petsc-users@xxxxxxxxxxx
- Subject: Re: Question regarding PCMG
- From: "Matthew Knepley" <knepley@xxxxxxxxx>
- Date: Mon, 13 Aug 2007 18:11:19 -0500
- Dkim-signature: a=rsa-sha1; c=relaxed/relaxed; d=gmail.com; s=beta; h=domainkey-signature:received:received:message-id:date:from:to:subject:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=AL2SDVVzxfud4wV/Popd0SeyGKB2TIHhkn9vGOfY+Pg1x2i7qdOL5Dzrhk7rp8L4UpBrTb6BtLpiMxTOYbb2VKIxOUXkYxZcvPTFSpaCykTJ4yFSqKPYr+8wZi3nca77G/zAXE/xCKDSf9T15KwXp+DFkk01T3K3BhE0nM2cB+I=
- Domainkey-signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=received:message-id:date:from:to:subject:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=lnx18kOHYeQSjKsSFeGZLVc9jzolgB9IZwxbqukM4zp5bGIZOqLPFuHzX8k1X0HDH8C/etpTGo4qlsCc32szFQNr9MibhTDjvC+/Hc3u7q/imuDCopSOazi3F/p7kQ9E0azZIP36I39YCOov3sKXrwv0xkmBFeDuHCOkPIauiuU=
- In-reply-to: <006501c7dde8$288c4120$96d28481@FUMBOLT>
- References: <006501c7dde8$288c4120$96d28481@FUMBOLT>
- Reply-to: petsc-users@xxxxxxxxxxx
- Sender: owner-petsc-users@xxxxxxxxxxx
On 8/13/07, TsuyoshiKoyama(berkeley) <tkoyama@xxxxxxxxxxxx> wrote:
> Dear petsc-users,
>
> I am currently trying to use the PCMG object and
> had some questions with the preconditioner.
>
> ---------------------------------------------------------
> 1. In this multigrid preconditioner, the default KSP smoother seems
> to be GMRES. If one wanted to use the Gauss-Seidel
> iteration as a smoother what would one do?
>
> The solution that I thought feasible is to use the PCGauss-Seidel
> and set this into the KSP smoother, and set the Krylov solver
> type to KSPPreonly. Since we are using a Gauss-Seidel smoothing
> and in the case that we include post-smoothing, the smoother must
> be able to incorporate a nonzero initial guess. Thus one would think
> of setting the flag for KSPSetInitialGuessNonzero. Unfortunately the
> combination of,
>
> -KSPPreonly
> -KSPSetInitialGuessNonzero
>
> is rejected in /src/ksp/ksp/impls/preonly/preonly.c line:24. Here it
> advises one to use a Richardson but that is not what I would like.
Actually, I believe Richardson is exactly what you want. It is just the simple
update.
> 2. In the setting of the coarse grid KSP smoother(solver) in PCMG, in the
> function
> PCSetUp_MG in file /src/ksp/pc/impls/mg/mg.c line:474 there is
> a setting that requires one to only be able to use lu, redundant, or
> cholesky as the solver in the case of preonly. This means that one would
> not again be able to use the type of solver that I have stated above,
> a Gauss-Seidel solver or any other user-specified stationary
> solver with KSPPreonly.
> ---------------------------------------------------------
I do not really understand what you want here. Can you give me more explanation?
The coarse problem by default is solved exactly.
Thanks,
Matt
> If you have any comments or tricks to get by this, it would be very
> helpful.
>
> Sincerely,
>
> -Tsuyoshi Koyama
--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which
their experiments lead.
-- Norbert Wiener