<div dir="ltr">Hi-<br>It is more matter of truncation at a certain number of processors specifically involving domain decomposition. That is using, say 64 processors, you will always get the same answer on a particular machine. But if you use 32 processors, the domain decomposition and subsequent integration produces slight differences probably at the level of truncation. Since all of our problems are highly non-linear any differences no matter how small changes the solution.<br>
<br>I have seen this issue before and even in small time period integrations. The key is the number of processors. I would imagine this is also compiler dependent (i.e. Exactly how functions are calculated and at what precision). I think differences in precipitation can be rather large when using the same data but using a different number of processors, especially when not a power of 2.<br>
<br>That is my understanding. If anyone else can correct me, or add more, I would be grateful as well.<br>jimmyc<br><br><div class="gmail_quote">On Thu, Sep 18, 2008 at 6:26 AM, Jan Ploski <span dir="ltr"><<a href="mailto:Jan.Ploski@offis.de">Jan.Ploski@offis.de</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Hello,<br>
<br>
WRF produces slightly different results depending on how many processors<br>
are used in a shared memory run (of course, the "slight" differences<br>
become bigger with integration time). I know that the general explanation<br>
for this is "rounding errors". What I would like to understand is why the<br>
rounding errors must appear at all. After all, data can be transmitted<br>
exactly from one MPI process to another, so why does it matter how many<br>
MPI processes are involved? I'd be grateful for a brief explanation...<br>
<br>
Regards,<br>
Jan Ploski<br>
<br>
--<br>
Dipl.-Inform. (FH) Jan Ploski<br>
OFFIS<br>
FuE Bereich Energie | R&D Division Energy<br>
Escherweg 2 - 26121 Oldenburg - Germany<br>
Phone/Fax: +49 441 9722 - 184 / 202<br>
E-Mail: <a href="mailto:Jan.Ploski@offis.de">Jan.Ploski@offis.de</a><br>
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</blockquote></div><br><br clear="all"><br>-- <br>James Correia Jr.<br>Post Doc<br>Climate Physics Group, PNNL<br><br>
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