[nmag-users] Hysteresis loops stalling and MPI convergence

[Graham Rowlands]

Greetings nmag users,

I've been observing a few strange behaviors lately and wanted to see if anyone had any insight. I have investigated the second problem more than the first at this point.

1.) Convergence for simple meshes (1k-8k nodes) sometimes fails with larger number of processors (~8) but appears consistently fine with fewer (~4). 

2.) Larger meshes (50k-100k nodes) get stuck in hysteresis loops. Occasionally I see errors from CVODE about convergence, but normally this occurs without any useful information being printed on INFO, INFO2, or DEBUG channels. The simulation may work fine for several field steps (with convergence=True), but then at some unpredictable step it prints 

      INFO hysteresis: starting new stage: field = [xx xx xx]

and sits there spinning its wheels indefinitely without any indication of trouble in the various log files. I'm doing the calculations with a large magnetic damping and have disabled the precessional term in the LLG eqn.

The meshes that tend to fail are usually somewhat complicated in shape, but I've spent a good amount of time attempting to ensure that they are of high quality. A zip file containing both vtk and nmesh.h5 formatted versions of an example mesh can be found at https://dl.dropboxusercontent.com/u/10789691/problem-mesh.zip. The features of the mesh are somewhat small compared to the exchange length, so perhaps there are some stiffness problems. Please note that I've seen problems with simpler meshes, but not for a completely flat mesh. Here is the output from nmeshpp -i -q from the files linked above:

====== Info output: =====================================

    3-dimensional mesh

43582 volume elements (3d)

12044 surface elements (2d)

10544 points

    1 simplex regions ([1])

    2 point regions ([-1, 1])

    2 region volumes ([0.0, 104.04922935933718])

 6024 boundary points (-> BEM size<=  277MB)

    0 periodic points (mirage=0, total=0)

a0: average=0.260762, std=0.093217, min=0.085937, max=0.774413

====== Quality output: ===================================

[qual interval] counts = probability

[ 0.000- 0.100]      0 = 0.00% 

[ 0.100- 0.200]      0 = 0.00% 

[ 0.200- 0.300]      1 = 0.00% *

[ 0.300- 0.400]      8 = 0.02% *

[ 0.400- 0.500]    103 = 0.24% *

[ 0.500- 0.600]    642 = 1.47% **

[ 0.600- 0.700]   3137 = 7.20% *******

[ 0.700- 0.800]   9923 =22.77% **********************

[ 0.800- 0.900]  18781 =43.09% ****************************************

[ 0.900- 1.000]  10987 =25.21% ************************

I'm curious if the problems are somehow related to my own installation, which is on Ubuntu 14.04. I had to update a few of the packages in order to successfully compile nmag, i.e. 

PYTHON_PKG=pkgs/Python-2.7.7.tar.gz
IPYTHON_PKG=pkgs/ipython-2.1.0.tar.gz
PYTHON_CYTHON_PKG=pkgs/Cython-0.20.1.tar.gz 
PYTHON_NUMEXPR_PKG=pkgs/numexpr-1.4.1.tar.gz
PYTHON_PYTABLES_PKG=pkgs/tables-2.4.0.tar.gz
PYTHON_NUMPY_PKG=pkgs/numpy-1.8.1.tar.gz
PYTHON_SCIPY_PKG=pkgs/scipy-0.14.0.tar.gz
PYTHON_PY_PKG=pkgs/py-0.9.1.tar.gz
PYTHON_PLY_PKG=pkgs/ply-3.3.tar.gz
SUNDIALS_PKG=pkgs/sundials-2.3.0.tar.gz
HDF5_PKG=pkgs/hdf5-1.8.13.tar.bz2

I suppose that SUNDIALS is implicated with the CVODE convergence issues. I believe choosing that particular version was necessary to maintain API compatibility. Soon I will be trying the nmag virtual machine image to see if it is indeed my own environment.

Other things I've tried that didn't work:

1.) Running without MPI

2.) Obsessing over the mesh quality

3.) Reordering the nodes with nmeshpp --reordernodes

4.) Decreasing tolerances 

So, it anyone has any good ideas for debugging steps I would be very grateful! I'm new to finite element calculations, so it's very possible that I've made some glaring errors.

Best and Thanks,

Graham

---

Graham Rowlands

Postdoctoral Research Associate

Buhrman Lab, Applied & Engineering Physics

Cornell University

Ithaca NY, USA