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<p class="MsoNormal">For question 2, there are some discussions in PETSc’s manual section 4.3 and 4.3.3.<o:p></o:p></p>
<p class="MsoNormal"><a href="https://www.mcs.anl.gov/petsc/petsc-current/docs/manual.pdf">https://www.mcs.anl.gov/petsc/petsc-current/docs/manual.pdf</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Ling<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">petsc-users <petsc-users-bounces@mcs.anl.gov> on behalf of Yingjie Wu via petsc-users <petsc-users@mcs.anl.gov><br>
<b>Reply-To: </b>Yingjie Wu <yjwu16@gmail.com><br>
<b>Date: </b>Monday, April 1, 2019 at 9:22 AM<br>
<b>To: </b>"petsc-users@mcs.anl.gov" <petsc-users@mcs.anl.gov><br>
<b>Subject: </b>[petsc-users] Problem about Residual evaluation<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Dear PETSc developers: <o:p></o:p></p>
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<p class="MsoNormal">Hi,<o:p></o:p></p>
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<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">I've been using -snes_mf_operator and I've customized a precondition matrix to solve my problem.I have two questions about the residuals of linear steps(KSP residual).<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap"><o:p> </o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">1.Since I'm using a matrix-free method, how do we get KSP residuals in PETSc?
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">r_m = b - A*x_m<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">Is finite difference used to approximate "A*x_m" ?<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap"><o:p> </o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">2.What is the difference between instruction ' -ksp_monitor ' and ' -ksp_monitor_true_residual ' in how they are calculated?
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap"><o:p> </o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">Thanks,<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap">Yingjie<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap"><o:p> </o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;white-space:pre-wrap"><o:p> </o:p></p>
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