I've encountered a very strange problem and I'm wondering if anyone else
has encountered it and/or whether I might be doing something wrong.
I am running LAMMPS built by these instructions:
https://www.alcf.anl.gov/user-guides/lammps
for a Blue Gene/Q system; note that you have to add the two "vpath" lines
that exist in the standard makefiles to make this work with recent LAMMPS
make scripts. Packages included are: MANYBODY, REPLICA, USER-MISC,
USER-OMP. It's built with the IBM XL C++ compiler for Blue Gene, version
12.1.
The problem can be reproduced with the standard "in.lj" benchmark, with
the following modifications:
* Lines 7-9 should multiply by 100, not 20
* Add the line "dump 1 all atom 1000 dump.lj" right before "run 100"
Then run with 512 nodes with 16 MPI ranks per node and 4 threads per rank.
It might be reproducible with fewer ranks, but that's the smallest job I
can run on this system. Anyway....
Now look at the dump file. If you see the same problem I do, it will have
normal-looking numbers at the top, but after a while all the lines will
look like
0 0 0 0 0
OR
0 0 1.0E-320 1.0E-320 1.0E-320
or some other garbage numbers. Then there will be a block of normal ones,
then more garbage. I should point out that the block of "normal" values
is almost always 128 entries long, which is very suspicious; the block of
zeros varies in length. Some entries also look like
5 1260512 1 1 61.3053
which is equally nonsensical.
It's worth noting that I can't reproduce this problem (at least not with
this particular benchmark file) with 20x20x20 or even 50x50x50 boxes.
100x100x100 does it, as does 200x200x200. As, of course, does the REAL
system I was trying to simulate....
Any ideas what might be going wrong or checks I can perform? It may be a
compiler issue, but at this point it's not 100% clear to me how to
determine that or how to fix it if it is.
Karl D. Hammond
University of Tennessee, Knoxville
[email protected]...
"You can never know everything, and part of what you know is always
wrong. A portion of wisdom lies in knowing that. A portion of courage
lies in going on anyway."
"Nothing ever goes as you expect. Expect nothing, and you will not be
surprised."