[xml] Freeing large documents extremely slow

["Edward Z. Yang" <edwardzyang thewritingpot com>]

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Over at phpdoc (the PHP Documentation Project), we regularly deal with
very large XML files that occupy approximately 400-500 MB in the RAM.
Lower-end, low RAM systems (the two sample ones we've seen were
single-core, with 256-512 MB physical memory, so they have to use the
swap partition.)

What appears to happen is that the allocation of memory goes reasonably
quickly, but then when the PHP libxml extension attempts to deallocate
the memory with xmlFreeDoc(), the system hangs, and we need to send a
SIGINT to terminate the process.

Any ideas why this may be happening? Also, would this be a bug in the
PHP XML bindings, or libxml itself?

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