Fun with Basic PHP Optimization

Jan 9, 2014   #php  #programming 

A while ago I came across a full-featured PHP application for controlling a daemon. It worked well with a small data set, but quickly became laggy with a dataset numbering in the thousands. Admittedly, it really wasn’t built for that kind of load, so I removed it and controlled the daemon manually, which wasn’t a big deal.

Then a while later, I came across a post by someone who managed to mitigate the problem by shifting a particularly expensive operation to an external python program. Obviously, this was not exactly the most elegant solution, so I decided to take a look at the problematic section of code.

It looked something like this:

<?php
for ($i = 0; $i<count($req->val); $i+=$cnt) {
  $output[$req->val[$i]] = array_slice($req->val, $i+1, $cnt-1);
}
?>

Looks pretty basic, right? It cycles through an array ($req) and splits it into a new dictionary ($output) based on a fixed length ($cnt). However, if we turn this into a generic big O structure, with the values borrowed from this serverfault post, the problem quickly becomes apparent.

<?php
for ($i = 0; $i<O(n); $i+=$cnt)
  $output[$req->val[$i]] = O(n)
?>

Taking into account the for loop, this would appear to mean that the operation is O(2n2), in contrast to the very similar array_chunk O(n) function. So how do we optimize this? The most important thing to do is make it so php can complete this in one loop over the array. Everything else will be a nice improvement, but when scaling, the big O is king.

Here’s the new code:

<?php
foreach($req->val as $index=>$value)
{
  if($index % $cnt == 0)
  {
    $current_index = $value;
    $output[$current_index] = array();
  }
  else
    $output[$current_index][] = $value;
}
?>

We’ve dropped the for/count() loop in favor of foreach, and eliminated slicing in favor of appending to newly created elements. In a real world test, this cut down the response time of the module from 12s to 4s on average. A pretty big improvement for a pretty small change!