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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@5503 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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sjplimp
2011-01-07 21:40:58 +00:00
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@ -327,6 +327,45 @@ build it. In the src/MAKE/Windows directory are some notes from users
on how they built LAMMPS under Windows, so you can look at their
instructions for tips. Good luck - we can't help you on this one.
</P>
<P>(5) Changing the size limits in src/lmptype.h
</P>
<P>If you are running a very large problem (billions of atoms or more)
and get a run-time error about the system being too big, either on a
per-processor basis or in total size, then you may need to change one
or more settings in src/lmptype.h and re-compile LAMMPS.
</P>
<P>As the documentation in that file explains, you have basically
two choices to make:
</P>
<UL><LI>set the data type size of integer atom IDs to 4 or 8 bytes
<LI>set the data type size of integers that store the total system size to 4 or 8 bytes
</UL>
<P>The default for atom IDs is 4-byte integers since there is a memory
and communication cost for 8-byte integers. Non-molecular problems do
not need atom IDs so this does not restrict their size. Molecular
problems (which use IDs to define molecular topology), are limited to
about 2 billion atoms (2^31) with 4-byte IDs. With 8-byte IDs they
are effectively unlimited in size (2^63).
</P>
<P>The default for total system size quantities (like the number of atoms
or timesteps) is 8-byte integers by default which is effectively
unlimited in size (2^63). If your system does not support 8-byte
integers, an error will be generated, and you will need to set
"bigint" to 4-byte integers. This restricts your total system size to
about 2 billion atoms or timesteps (2^31).
</P>
<P>Note that in src/lmptype.h there are also settings for the MPI data
types associated with the integers that store atom IDs and total
system sizes, which need to be set consistent with the associated C
data types.
</P>
<P>In all cases, the size of problem that can be run on a per-processor
basis is limited by 4-byte integer storage to about 2 billion atoms
per processor (2^31), which should not normally be a restriction since
such a problem would have a huge per-processor memory footprint due to
neighbor lists and would run very slowly in terms of CPU
secs/timestep.
</P>
<HR>
<H4><A NAME = "2_3"></A>2.3 Making LAMMPS with optional packages