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Thank you for your answer!" Did you set DEAL_II_NUM_THREADS=1?"How can I double-check that?ccmake .only shows my the variables CMAKE_BUILD_TYPE and deal.II_DIR .But I do do knot if this is the right place to look for.
" That could explain why CPU and Wall time are different. Finally, if I understand correctly, you are calling the constructor of FEValues about 2.5 million times. That means that the call to one FEValues constructor is 100/2.5e6 seconds about 40 microseconds. That doesn't seem too slow. "There was a typo in my post. It should be 160/2.5e6 seconds about 64 microsecends.
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Dear Simon,
When you use FEPointEvaluation, you should construct it only once
and re-use the same object for different points. Furthermore, you
should also avoid to create "p_dofs" and the "std::vector" near
the I was not clear with my original message. Anyway, the problem
is the FEValues object that gets used. I am confused by your other
message that you use FE_Q together with MappingQ - that
combination should be supported and if it is not, we should take a
look at a (reduced) code from you.
Regarding the high timings: There is some parallelization by tasks that gets done inside the constructor of FEValues. This has good intents for the case that we are in 3D and have a reasonable amount of work to do. However, you are in 1D (if I read your code correctly), and then it is having adverse effects. The reason is that the constructor of FEValues is very likely completely dominated by memory allocation. When we have 1 thread, everything is fine, but when we have multiple threads working they will start to interfere with each other when the request memory through malloc(), which has to be coordinated by the operating system (and thus gets slower). In fact, the big gap between compute time and wall time shows that there is a lot of time wasted by "system time" that does not do actual work on the cores.
I guess the library could have a better measure of when to spawn tasks in FEValues in similar context, but it is a lot of work to get this right. (This is why I keep avoiding it in critical functions.)
Best,
Martin
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