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Hi,<br>
<br>
Sorry for small offtopic.<br>
I would also be interested more to know how it scales system? I mean
what is idea behind this scaling.<br>
<br>
Regards,<br>
Alexander<br>
<br>
On 23.12.2011 03:50, Mohamad M. Nasr-Azadani wrote:
<blockquote
cite="mid:CAE_bc5aZt-qGNN21UnUgnZOP2x5sKS-kJ5Sp1t2oaYiRPDTqiA@mail.gmail.com"
type="cite">Hi guys,
<div><br>
</div>
<div>Three questions: </div>
<div><span style="font-family: 'Times New Roman';
background-color: rgb(255, 255, 255); font-size: medium;">1)
As mentioned on the web when we use </span></div>
<div><b style="font-family: 'Times New Roman'; background-color:
rgb(255, 255, 255); font-size: medium;">-ksp_diagonal_scale_fix: </b><span
class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;">scale the matrix back AFTER the solve</span></div>
<div><span class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;"><br>
</span></div>
<div><span class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;">with -ksp_diagonal_scale d</span><span
class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;">oes it also scale back the RHS vector associated with
the KSP solver? </span></div>
<div><span class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;"><br>
</span></div>
<div><span class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;">2) Also, as mentioned on the web for
-ksp_diagonal_scale</span></div>
<div><span class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;">"Tells </span><a moz-do-not-send="true"
href="http://www.mcs.anl.gov/petsc/petsc-3.1/docs/manualpages/KSP/KSP.html#KSP"
style="font-family: 'Times New Roman'; background-color:
rgb(255, 255, 255); font-size: medium;">KSP</a><span
class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;"> to symmetrically diagonally scale the system before
solving. "</span></div>
<div><span class="Apple-style-span" style="font-family: 'Times New
Roman'; background-color: rgb(255, 255, 255); font-size:
medium;"><br>
</span></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">When I used this option to solve for the Pressure
(Poisson equation with Neumann B.C., descritized via
structured fisytenite-difference method), the number of
iterations for convergence was trippled, e.g. without this
option, it converges in 18 iteration, but with this option it
goes up to 50-60 iterations. </font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">(GMRES + BoomerAMG as the preconditioner). </font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">Is this something you would expect? </font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'"><br>
</font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">3) How could the linear system resulting from the
Poisson equation for pressure (incompressible flow) can be
more diagonally scaled? To elaborate more, for the uniform
grid case and let's say a second order central scheme finite
difference discretization, the sum of the off-diagonal
coefficients are equal to the diagonal coefficient. </font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'"><br>
</font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">Thanks, </font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">Best, </font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'">Mohamad</font></div>
<div><font class="Apple-style-span" size="3" face="'Times New
Roman'"><br>
</font></div>
</blockquote>
<br>
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