About Improving Performance When Using Xrefs

About Improving Performance When Using Xrefs

Unload Xrefs in Large Drawings

When a referenced drawing (xref) is unloaded from the current drawing, the drawing
opens much faster and uses less memory.

The xref definition is unloaded from the drawing file, but the internal pointer to
the referenced drawing remains. The xref is not displayed, and nongraphical object
information does not appear in the drawing. However, you can restore all the information
by reloading the xref. If XLOADCTL (demand loading) is set to 1, unloading the drawing
unlocks the original file.

You should unload a reference file if it is not needed in the current drawing session
but may be used later for plotting. You can maintain a working list of unloaded xrefs
in the drawing file that you can load as needed.

Overview of Demand Loading

The program uses demand loading and saving drawings with internal indexes to increase performance with large referenced
drawings that have been clipped, or that have many objects on frozen layers. With
demand loading, only the data from the reference drawing that is necessary to regenerate
the current drawing is loaded into memory. In other words, referenced data is read
in “on demand.â€‌

Demand loading works in conjunction with the INDEXCTL, XLOADCTL, and XLOADPATH system

To maximize the benefits of demand loading, save referenced drawings with layer and
spatial indexes. The performance benefits of demand loading are most noticeable when
you do one of the following:

  • Clip the xref to display a small fraction of it. A spatial index is saved in the
    externally referenced drawing.
  • Freeze several layers of the xref. The externally referenced drawing is saved with
    a layer index.

If demand loading is turned on, and you have clipped xrefs with spatial indexes,
objects in the referenced drawing database contained within the clip volume comprise
the majority of the objects read into the drawing. If the clip volume is modified
more objects are loaded as needed from the reference drawing. Similarly, if you have
xrefs with many layers frozen that were saved with layer indexes, only the objects
on those thawed layers are read into the current drawing. If those xref-dependent
layers are thawed, the program reads in that geometry from the reference drawing as

When demand loading is enabled, the program places a lock on all reference drawings
so that it can read in any geometry it needs to on demand. Other users can open those
reference drawings, but they cannot save changes to them. If you want other users
to be able to modify an xref that is being demand loaded into another drawing, use
demand loading with the Copy option.

If you turn on demand loading with the Enable with Copy option, the program makes
a temporary copy of the referenced drawing and demand loads the temporary file. You
can then demand load the xref while allowing the original reference drawing to be
available for modification. When you turn off demand loading, the program reads in
the entire reference drawing regardless of layer visibility or clip instances.

Layer and spatial indexes were added in AutoCAD Release 14 and AutoCAD LT 97. If
you externally reference a drawing saved in a release previous to this, you do not
see the same performance benefit as drawings saved with the indexes. For maximum performance,
use demand loading with referenced drawings saved with layer and spatial indexes turned
on in AutoCAD Release 14, AutoCAD LT 97, or more recent versions.

Learning AutoCad

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