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Mapping of exodus entities onto NetCDF

The distributed version of netcdf sets the following limits on dimensions and variables:

  • #define NC_MAX_DIMS 1024
  • #define NC_MAX_VARS 8192

For use with Exodus, it is recommended that these be increased to:

  • #define NC_MAX_DIMS 65536
  • #define NC_MAX_VARS 524288

The reason for these increases is due to the mapping of Exodus onto NetCDF. The sections below show the number of Dimensions (controlled by NC_MAX_DIMS) and Variables (controlled by NC_MAX_VARS) that are used in an Exodus file.

Entities

  • A mesh-entity is an individual node, edge, face, or element.
  • An entity is a set or block consisting of a single mesh-entity type.
  • Each entity can have variables, maps, and attributes which contain an entry per mesh-entity.
  • Each entity has an optional name and a required id (32-bit or 64-bit )which is non-negative.
  • A mesh-entity can be in one and only one entity block,
  • A mesh-entity can be in zero or more entity sets.
  • Currently there is only a single implicit node block containing all nodes in the model.

Dimensions: (NC_MAX_DIMS)

  • There are about 10 standard dimensions in every file.
  • plus one for each set plus one if any attributes
  • plus two for each block plus one if any attributes
  • plus one for each transient variable on an entity (node, node set, element block, element set, ...)

Variables: (NC_MAX_VARS)

  • There are a few standard dimensions

    • times
    • names of each entity type (block set)
    • ids of each entity type (block set)
    • status of each entity type (block set)
    • #ndim coordinates (1,2,3)

  • Each block adds 1 + 2*#attr_on_block + #var_on_block

  • Each set adds 2 + 2*#attr_on_set + #var_on_set

  • Each sideset add 3 + 2*#attr_on_sset + #var_on_sset

  • Each map adds 1

Example

If we have an exodus file with:

  • Nodes
  • 5 Element blocks
    • 4 transient variables per element block
    • 2 attributes per element block
  • 4 Node Sets
    • Distribution Factors defined on each set
    • 3 transient variables
  • 3 Side Sets
    • Distribution Factors defined on each set
    • 2 transient variables

Then there would be about: 10 + 5*(2+1) + 4*(2) + 3*(2) + 1 + 1 + 1 = 42 Dimensions

There would be about: 5*(1+2*2+4) + 4*(2+3) + 3*(3+2) + 3*(5+4+3) + 3 + 1 = 120 Varibles.

From this, you can see that a moderately complicated model would quickly overflow the standard values for NC_MAX_DIMS and NC_MAX_VARS.