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From David Bertoni <dbert...@apache.org>
Subject Re: Right usage of xerces
Date Tue, 21 Jul 2009 18:04:43 GMT
Altman, Drora wrote:
> Hi,
> I tried to do what suggested to me, adopted the document and released the parser, but
it didn't help much to reduce the memory consumption.
> So I made the following trials which didn't succeed too much. Could you please help me
figure out what I'm doing wrong and how should I do it correctly?
> 1. I tried to import the root node and release the document.
>         After the call to DOMDocumentImpl::release() some memory is released, but:
>         a. not all memory (used memory is more than what was before starting to use xerces)
This could be memory being held by the C++ run-time heap, which will be 
used for later allocations.  You might check your pl

>         b. after releasing the memory, I couldn't use the root node anymore since it's
corrupted after the document's released.
Yes, because releasing the document destroys everything.  What I 
suggested you do to compact a document is to clone the entire document:

newdoc = static_cast<DOMDocument*>(doc->cloneNode(true));
doc = newdoc;

> 2. I saw that there is an adoptNode() function, so I tried to use it- adopt the root
node and than release
>    the document.
>         However, when trying this I got an exception, so I looked at the source code
and found that in xerces 2.8.0 adoptNode() has the following implementation:
> DOMNode* DOMDocumentImpl::adoptNode(DOMNode*) {
>     throw DOMException(DOMException::NOT_SUPPORTED_ERR, 0, getMemoryManager());
>     return 0;
> }
> I looked at the source code of xerces 3.0.1 which has different implementation to that
function, however, it didn't seem to do what I expected it to do (bottom line is, that using
it didn't help either)
adoptNode() is used to bring a node in from another document, which 
Xerces-C doesn't support.

> - Eventually, I don't know how can I release the memory allocated by xerces....
You usually cannot control on the C++ run-time heap relinquishes memory 
to the operating system. You might want to investigate what sort of heap 
control and debugging functions exist on your platform to see if there 
are diagnostics that will help you figure out memory usage.

> Another issue that I realized is:  I saw that we use (quite a lot) the function:
> DOMNodeList *DOMElementImpl::getElementsByTagName(const XMLCh *tagname) const
> I wondered when (and by whom) should the returned DOMNodeList* be deleted, I saw an answer
regarding this issue in the following link:
> http://www.mail-archive.com/c-dev@xerces.apache.org/msg02511.html
> but didn't understand what is the meaning of: Memory for any returned object ... are
owned by implementation - whose implementation?
> I think that releasing these lists after we used finished using them might help us as
well (I saw that the fNodeListPool is cleaned up when the document is deleted, but it is not
explicitly deleted in any place I could find)
These are kept in a pool within the document, so you cannot release 
them. If you are calling this often with different tag names, that could 
explain some increased memory usage.  You might want to file an 
enhancement request that allows for clearing this pool. If you use the 
approach I outlined of cloning the entire document, that will take care 
of cleaning up this pool.


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