Hello everyone
First I want to say, it is so much easier and more pleasant to work with
PETScWrappers in dealii than writing PETSc code in C. Developing finite
element code based on dealii is 1000 times more efficient than writing from
scratch!
Recently I parallelized a time-dependent linear elasticity code with
PETScWrappers, based on step-40. I tested its performance on my desktop.
But I am not sure if the result is "normal".
The test case is the bending of a 3D cantilever beam. There are 262144
active cells and 839619 dofs. I ran 2 time steps, which involves two
assembling and solving calls, on 1, 2, 4, 8 processors, respectively. The
wall time looks like this:
| assemble (s) | solve (s) | total (s)
-------------------------------------------------------------------
n = 1 | 274.0 | 44.44 | 362.0
n = 2 | 140.50 | 27.54 | 192.0
n = 4 | 75.72 | 17.12 | 106.3
n = 8 | 64.74 | 16.62 | 92.82
The speedup from n = 1 up to n = 4 is quite obvious, but from n = 4 to n =
8, it is insignificant. Actually sometimes running with 8 ranks is even
slower than
running with 4 ranks. My desktop has one intel i7-6700k cpu, which has 4
cores but 8 threads, and 16GB memory. I do not quite understand the
difference between "thread" and "rank". Should I expect the performance to
scale up to 4 or 8 mpi ranks?
Another general question is that: in distributed parallel applications, I
use temporary objects to copy a non-ghosted vector to a ghosted vector, or
vice versa all the time. For example, I use non-ghosted vector to store my
solution, but have to copy it to a ghosted vector when I output results or
refine mesh. On the other hand, if I use ghosted vector to store my
solution, I have to copy it to a non-ghosted vector when I manipulate it
with PETScWrappers::VectorBase::add (for example subtracting time
discretization terms from it).
I wanna ask, is this copy operation expensive? Is there a way to avoid that?
Thank you
Jie
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