FOP Memory Usage

[Keiron Liddle <ke...@aftexsw.com>]

Just to give everyone some information about the memory issue.

All previous releases of FOP have done the following:
- parse the xml file
- build an object hierarchy representing the xml
- convert these objects into an area tree
- render the area tree

This means that in memory there is a full object representation of the xml
document, a full area tree of the output document and a full rendered
document at the same time plus overhead. Obviously this tends to use a lot
of memory. If you use dom then you will also have the xml dom document in
memory, FOP simply uses it as SAX anyway.

This was recently altered (0.20 I think) so that the area tree was kept
only of page sequences and some of the output document was written
immediately. This improved it depending on your documents but was not a
complete solution.

The problem is mainly design, the original design is from quite a while ago
and was never changed to handle the situation better. So now we are
attempting to redesign FOP so that memory use is better along with the more
important issue of layout.

This redesign will probably take at least 6 more months to complete at the
current rate.


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