On 2/9/22 01:49, Mohammad Fazli wrote:
I am using dealii and I faced a big problem with parallel scaling on
ubuntu 18.04.
For instance, I have compiled step-17 with different core numbers on my
machine:
Memory: 31.2 GB,
Processor: Intel® Core™ i7-8700 CPU @ 3.20GHz × 12,
which shows me:
*time mpirun -np 1 ./step-17*
*real 0m17.701s*
*time mpirun -np 2 ./step-17*
*real 0m16.507s*
*time mpirun -np 4 ./step-17*
*real 0m33.934s*
*
*
I have tried to install dealii several times but all the time the same
issue happens. Someone told me the problem can be with p4est. But I
don't know what I should do to resolve this issue.
Mohammad: step-17 does not actually use p4est, so that is not the cause.
Your results match what I see on my machine reasonably closely -- this
is what I get here:
1 processors: 0m25.833s
2 processors: 0m20.885s
4 processors: 0m14.838s
8 processors: 0m15.463s
16 processors: 0m15.858s
32 processors: 0m17.176s
In other words, there is not very much speedup. There can be many
reasons for that, among which are:
* You have only a single CPU. Using multiple MPI jobs will eventually
run into memory bandwidth limitations between the CPU and main memory.
* The problem is quite small (on the finest level, step-17 has only
about 40,000 unknowns). Breaking it into even smaller pieces does not
leave very much work for each MPI process, but requires substantial
communication.
* The program is intended to demonstrate some basic functionality, but
it is not tuned for efficiency, and certainly not for such small problems.
So there may not actually be anything wrong with what you do. If you
wanted to have a more realistic test, make sure that you increase the
initial number of refinement steps so that you have several 100,000
unknowns, and then try again with that.
Best
W.
--
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Wolfgang Bangerth email: [email protected]
www: http://www.math.colostate.edu/~bangerth/
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