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<H1><A NAME="SECTION00090000000000000000"></A><A NAME="sec:adding"></A>
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<BR>
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Adding new smoother and solver objects to MLD2P4
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</H1>
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<P>
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Developers can add completely new smoother and/or solver classes
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derived from the base objects in the library (see Remark 2 in Section <A HREF="node17.html#sec:precset">6.2</A>),
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without recompiling the library itself.
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<P>
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To do so, it is necessary first to select the base type to be extended.
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In our experience, it is quite likely that the new application needs
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only the definition of a ``solver'' object, which is almost
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always acting only on the local part of the distributed matrix.
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The parallel actions required to connect the various solver objects
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are most often already provided by the block-Jacobi or the additive
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Schwarz smoothers. To define a new solver, the developer will then
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have to define its components and methods, perhaps taking one of the
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predefined solvers as a starting point, if possible.
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<P>
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Once the new smoother/solver class has been developed, to use it in
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the context of the multilevel preconditioners it is necessary to:
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<UL>
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<LI>declare in the application program a variable of the new type;
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</LI>
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<LI>pass that variable as the argument to the <code>set</code> routine as in the
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following:
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<DIV ALIGN="CENTER">
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<code>call p%set(smoother,info [,ilev,ilmax,pos])</code>
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<BR><code>call p%set(solver,info [,ilev,ilmax,pos])</code>
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</DIV>
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</LI>
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<LI>link the code implementing the various methods into the application executable.
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</LI>
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</UL>
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The new solver object is then dynamically included in the
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preconditioner structure, and acts as a <I>mold</I> to which the
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preconditioner will conform, even though the MLD2P4 library has not
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been modified to account for this new development.
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<P>
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It is possible to define new values for the keyword <code>WHAT</code> in the
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<code>set</code> routine; if the library code does not recognize a keyword,
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it passes it down the composition hierarchy (levels containing
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smoothers containing in turn solvers), so that it can be eventually caught by
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the new solver.
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<P>
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An example is provided in the source code distribution under the
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folder <code>tests/newslv</code>. In this example we are implementing a new
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incomplete factorization variant (which is simply the ILU(0)
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factorization under a new name). Because of the specifics of this case, it is
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possible to reuse the basic structure of the ILU solver, with its
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L/D/U components and the methods needed to apply the solver; only a
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few methods, such as the description and most importantly the build,
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need to be ovverridden (rewritten).
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<P>
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The interfaces for the calls shown above are defined using
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<DIV ALIGN="CENTER">
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<TABLE CELLPADDING=3>
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<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=40><code>smoother</code></TD>
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<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=340><code>class(mld_x_base_smoother_type)</code></TD>
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</TR>
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<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=40> </TD>
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<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=340>The user-defined new smoother to be employed in the
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preconditioner.</TD>
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</TR>
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<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=40><code>solver</code></TD>
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<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=340><code>class(mld_x_base_solver_type)</code></TD>
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</TR>
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<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=40> </TD>
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<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=340>The user-defined new solver to be employed in the
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preconditioner.</TD>
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</TR>
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</TABLE>
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</DIV>
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The other arguments are defined in the way described in
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Sec. <A HREF="node17.html#sec:precset">6.2</A>. As an example, in the <code>tests/newslv</code>
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code we define a new object of type <code>mld_d_tlu_solver_type</code>, and
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we pass it as follows:
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<PRE>
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! sparse matrix and preconditioner
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type(psb_dspmat_type) :: a
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type(mld_dprec_type) :: prec
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type(mld_d_tlu_solver_type) :: tlusv
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......
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!
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! prepare the preconditioner: an ML with defaults, but with TLU solver at
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! intermediate levels. All other parameters are at default values.
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!
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call prec%init('ML', info)
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call prec%hierarchy_build(a,desc_a,info)
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nlv = prec%get_nlevs()
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call prec%set(tlusv, info,ilev=1,ilmax=max(1,nlv-1))
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call prec%smoothers_build(a,desc_a,info)
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</PRE>
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