<div dir="ltr"><div dir="ltr">On Thu, Feb 18, 2021 at 7:15 PM Barry Smith <<a href="mailto:bsmith@petsc.dev">bsmith@petsc.dev</a>> wrote:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><br><div><br><blockquote type="cite"><div>On Feb 18, 2021, at 6:10 AM, Matthew Knepley <<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>> wrote:</div><br><div><div dir="ltr"><div dir="ltr">On Thu, Feb 18, 2021 at 3:09 AM Roland Richter <<a href="mailto:roland.richter@ntnu.no" target="_blank">roland.richter@ntnu.no</a>> wrote:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div><p>Hei,</p><p>that was the reason for increased run times. When removing
#pragma omp parallel for, my loop took ~18 seconds. When changing
it to #pragma omp parallel for num_threads(2) or #pragma omp
parallel for num_threads(4) (on a i7-6700), the loop took ~16 s,
but when increasing it to #pragma omp parallel for num_threads(8),
the loop took 28 s.</p><div><br></div></div></blockquote><div>Editorial: This is a reason I think OpenMP is inappropriate as a tool for parallel computing (many people disagree). It makes resource management</div><div>difficult for the user and impossible for a library.</div></div></div></div></blockquote><div><br></div> It is possible to control these things properly with modern OpenMP APIs but, like MPI implementations, this can require some mucking around a beginner would not know about and the default settings can be terrible. MPI implementations are not better, their default bindings are generally horrible. </div></div></blockquote><div><br></div><div>MPI allows the library to understand what resources are available and used. Last time we looked at it, OpenMP does not have such</div><div>a context object that gets passed into the library (comm). The user could construct one, but then the "usability" of OpenMP fades away.</div><div><br></div><div> Matt</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div> Barry</div><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div><br></div><div> Thanks,</div><div><br></div><div> Matt</div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><p>Regards,</p><p>Roland<br>
</p>
<div>Am 17.02.21 um 18:51 schrieb Matthew
Knepley:<br>
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<div dir="ltr">Jed, is it possible that this is an
oversubscription penalty from bad OpenMP settings? <said by a
person who knows less about OpenMP than cuneiform>
<div><br>
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<div> Thanks,</div>
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<div> Matt</div>
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<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, Feb 17, 2021 at 12:11
PM Roland Richter <<a href="mailto:roland.richter@ntnu.no" target="_blank">roland.richter@ntnu.no</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<div>My PetscScalar is complex double (i.e. even higher
penalty), but my matrix has a size of 8kk elements, so
that should not an issue.<br>
Regards,<br>
Roland
<hr style="display:inline-block;width:98%">
<div id="gmail-m_5563990973605368928gmail-m_3012486240833435834gmail-m_2709721564415767467x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri,
sans-serif"><b>Von:</b> Jed Brown <<a href="mailto:jed@jedbrown.org" target="_blank">jed@jedbrown.org</a>><br>
<b>Gesendet:</b> Mittwoch, 17. Februar 2021 17:49:49<br>
<b>An:</b> Roland Richter; PETSc<br>
<b>Betreff:</b> Re: [petsc-users] Explicit linking to
OpenMP results in performance drop and wrong results</font>
<div> </div>
</div>
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<font size="2"><span style="font-size:10pt">
<div>Roland Richter <<a href="mailto:roland.richter@ntnu.no" target="_blank">roland.richter@ntnu.no</a>>
writes:<br>
<br>
> Hei,<br>
><br>
> I replaced the linking line with<br>
><br>
> //usr/lib64/mpi/gcc/openmpi3/bin/mpicxx
-march=native -fopenmp-simd<br>
> -DMKL_LP64 -m64<br>
>
CMakeFiles/armadillo_with_PETSc.dir/Unity/unity_0_cxx.cxx.o
-o<br>
> bin/armadillo_with_PETSc <br>
>
-Wl,-rpath,/opt/boost/lib:/opt/fftw3/lib64:/opt/petsc_release/lib<br>
> /usr/lib64/libgsl.so /usr/lib64/libgslcblas.so
-lgfortran <br>
> -L${MKLROOT}/lib/intel64 -Wl,--no-as-needed
-lmkl_intel_lp64<br>
> -lmkl_gnu_thread -lmkl_core -lgomp -lpthread -lm
-ldl<br>
> /opt/boost/lib/libboost_filesystem.so.1.72.0<br>
> /opt/boost/lib/libboost_mpi.so.1.72.0<br>
> /opt/boost/lib/libboost_program_options.so.1.72.0<br>
> /opt/boost/lib/libboost_serialization.so.1.72.0<br>
> /opt/fftw3/lib64/libfftw3.so
/opt/fftw3/lib64/libfftw3_mpi.so<br>
> /opt/petsc_release/lib/libpetsc.so<br>
> /usr/lib64/gcc/x86_64-suse-linux/9/libgomp.so<br>
> /<br>
><br>
> and now the results are correct. Nevertheless,
when comparing the loop<br>
> in line 26-28 in file test_scaling.cpp<br>
><br>
> /#pragma omp parallel for//<br>
> // for(int i = 0; i < r_0 * r_1; ++i)//<br>
> // *(out_mat_ptr + i) = (*(in_mat_ptr + i)
* scaling_factor);/<br>
><br>
> the version without /#pragma omp parallel/ for is
significantly faster<br>
> (i.e. 18 s vs 28 s) compared to the version with
/omp./ Why is there<br>
> still such a big difference?<br>
<br>
Sounds like you're using a profile to attribute time?
Each `omp parallel` region incurs a cost ranging from
about a microsecond to 10 or more microseconds
depending on architecture, number of threads, and
OpenMP implementation. Your loop (for double
precision) operates at around 8 entries per clock
cycle (depending on architecture) if the operands are
in cache so the loop size r_0 * r_1 should be at least
10000 just to pay off the cost of `omp parallel`.<br>
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-- <br>
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<div>What most experimenters take for granted before
they begin their experiments is infinitely more
interesting than any results to which their
experiments lead.<br>
-- Norbert Wiener</div>
<div><br>
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<div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>-- Norbert Wiener</div><div><br></div><div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br></div></div></div></div></div></div></div></div>
</div></blockquote></div><br></div></blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>-- Norbert Wiener</div><div><br></div><div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br></div></div></div></div></div></div></div></div>