XML XPath搜索和阵列与PHP的循环，内存问题

问题描述

我处理大型XML文件（数兆字节），为此我不得不做出各种实物检查。不过，我有这非常快速增长的内存和时间的使用问题。我测试过这样的：

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;
}

使用microtime中（）和memory_get_usage（）-functions我得到这个运行code以下结果：

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

• 5MB的文件（7480 AMT的元素）：
  
  
  
  • 执行时间0,69s
  
  
  • 内存使用率从10.25Mb成长为29.75Mb
  
  

  这仍然是相当好的。但后来有一个大一点的文件记忆和使用时间增长非常

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

  
  
  • 6MB文件（8976 AMT的元素）：
    
    
    
    • 执行时间8,53s
    
    
    • 内存使用率从10.25Mb成长为99.25Mb
    
    

    这个问题似乎是在循环的结果集。我也试过循环代替的foreach但没有任何区别。如果没有循环内存使用量不会增长这么多。

    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.

    任何想法，问题可能是什么？

    Any idea where the problem could be?

    推荐答案

    SimpleXML的是基于树的，将整个文件加载到内存中。使用取消设置标记不再需要的的进行清理might产量较少的内存使用。如果那不解决这一问题，可以考虑使用为基于拉做法。虽然你将无法使用XPath，内存的消耗应该是显著下降。

    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.

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