XML xpath搜索和数组循环与PHP，内存问题

Question

I'm dealing with large XML files (several megabytes) for which I have to make various kind of checks. However I have problem with memory and time usage which grows very quickly. I've tested it like this:

$xml = new SimpleXMLElement($string);
$sum_of_elements = (double)0.0;

foreach ( $xml->xpath('//Amt') as $amt ) {
  $sum_of_elements += (double)$amt;
}

With microtime() and memory_get_usage() -functions I get the following results by running this code:

• 5Mb file (7480 Amt-elements):
  • execution time 0,69s
  • Memory usage grows from 10.25Mb to 29.75Mb

That's still quite ok. But then with a bit bigger file memory and time usage grow very much:

• 6Mb file (8976 Amt-elements):
  • execution time 8,53s
  • Memory usage grows from 10.25Mb to 99.25Mb

The problem seems to be in looping the result set. I've also tried for-loop instead of foreach but with no difference. Without looping the memory usage does not grow so much.

Any idea where the problem could be?

Answer 1

SimpleXML is tree-based and will load the entire document into memory. Using unset to mark no longer needed resources for PHP's GC for cleanup during a loop might yield less memory usage. If that doesnt solve the issue, consider using XMLReader for a pull-based approach. Though you won't be able to use XPath, memory consumption should be significantly lower.