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<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt">Dear SasView Users,</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt"> </span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white">We have become aware of
<b>an issue in the latest SasView 5.0.5</b> with the new Levenberg-Marquardt (LM) optimiser – the default optimiser -
<b>when you are fitting with a GPU enabled</b>. Specifically, if the model you are fitting against contains C code, either embedded in the Python model file or in an ancillary C model file, then nothing happens when you click Fit! The parameters in the FitPage
do not update and the fit to the data does not change. You must specifically enable GPU computations, if required, else they are performed on the CPU. So this is a very obvious bug and the chances that you have obtained incorrect results as a consequence are
very low.</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt"><br>
<span style="background:white">In contrast, if you are only using the CPU (the default condition), or if the model you are fitting is a pure Python model, then this new LM optimiser still works as expected. And if you are using an earlier version of SasView
then the old LM optimiser works with a GPU.</span></span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white"> </span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white">But if you need to use a GPU for your fitting in SasView 5.0.5 then the workaround is simply to select a different optimiser from Fitting > Fit Algorithms. See
<a href="https://www.sasview.org/docs/user/qtgui/Perspectives/Fitting/optimizer.html" target="_blank">
https://www.sasview.org/docs/user/qtgui/Perspectives/Fitting/optimizer.html</a> for more information.</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white"> </span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white">For the technically-minded, this issue has arisen because we changed the source library for the LM optimiser from SciPy to MPFit. We did this because the SciPy version did not respect
parameter bounds during fitting; something which several of you had complained about! Unfortunately, the fact that the MPFit version did not support parallelisation escaped our notice!</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white"> </span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white">To tell if your model uses C code, go to
</span><span lang="EN-US" style="font-size:11.0pt"><a href="https://github.com/SasView/sasmodels/tree/master/sasmodels/models" target="_blank">https://github.com/SasView/sasmodels/tree/master/sasmodels/models</a>. If there are .py and .c files with the same
filename then the model will be affected by this bug. This is, unfortunately, the majority of the library models.</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt"> </span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white"> </span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="SV" style="font-size:11.0pt;background:white">The SasView Development Team</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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