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Bug #71153 Performance Degradation in ArrayIterator with large arrays
Submitted: 2015-12-17 19:28 UTC Modified: 2015-12-17 19:47 UTC
Votes:1
Avg. Score:3.0 ± 0.0
Reproduced:1 of 1 (100.0%)
Same Version:1 (100.0%)
Same OS:0 (0.0%)
From: kontakt at beberlei dot de Assigned: nikic
Status: Closed Package: *General Issues
PHP Version: 7.0.1 OS: Linux/Ubuntu 15.04
Private report: No CVE-ID:
 [2015-12-17 19:28 UTC] kontakt at beberlei dot de
Description:
------------
When creating array iterators with large array, iterating over them will be *much* slower than just iterating over the array itself. The larger the array gets the exponentially slower the iteration gets. Memory usage increases as well.

For the same script PHP 5 doesn't have the problem, it is pretty fast on any size of array iterator.

This problem was found in Twig, see my explanation comment: https://github.com/twigphp/Twig/issues/1810



Test script:
---------------
<?php

class Node implements IteratorAggregate
{
    private $nodes;

    public function __construct(array $nodes)
    {
        $this->nodes = $nodes;
    }

    public function getIterator()
    {
        return new ArrayIterator($this->nodes);
    }

    public function getNodes()
    {
        return $this->nodes;
    }

    public function visitIterator()
    {
        foreach ($this as $node) {
            $node->visitIterator();
        }
    }

    public function visitNodes()
    {
        foreach ($this->getNodes() as $node) {
            $node->visitNodes();
        }
    }
}

gc_disable();

$sizes = [1000, 10000, 20000, 65000, 66000, 100000];

foreach ($sizes as $size) {
    echo "Array Size $size\n";
    $nodes = [];
    for ($i = 0; $i < $size; $i++) {
        $nodes[] = new Node([]);
    }
    $node = new Node($nodes);

    $time = microtime(true);
    $node->visitNodes();
    echo "Array: " . number_format(microtime(true) - $time , 4) . " " . memory_get_peak_usage() . "\n";

    $time = microtime(true);
    $node->visitIterator();
    echo "Iterator: " . number_format(microtime(true) - $time , 4) . " " . memory_get_peak_usage() . "\n";
    echo "\n";
}

Expected result:
----------------
PHP 7 being faster than PHP 5 on every size of ArrayIterator, not just with size 1000.

Actual result:
--------------
Array Size 1000
Array: 0.0001 695232
Iterator: 0.0009 730208

Array Size 10000
Array: 0.0019 2464128
Iterator: 0.0607 3003872

Array Size 20000
Array: 0.0027 5898752
Iterator: 0.5189 5898752

Array Size 65000
Array: 0.0130 14741656
Iterator: 5.0322 14741656

Array Size 66000
Array: 0.0107 24079792
Iterator: 12.1315 24079792

Array Size 100000
Array: 0.0141 32198240
Iterator: 34.8082 32198240


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 [2015-12-17 19:47 UTC] nikic@php.net
-Assigned To: +Assigned To: nikic
 [2015-12-17 19:49 UTC] kontakt at beberlei dot de
Ok it problem seems iteration over an empty ArrayIterator,

removing the recursive call $node->visitIterator(); inside the method makes php7 implementation faster than the php5 one.
 [2015-12-17 20:03 UTC] nikic@php.net
Automatic comment on behalf of nikic
Revision: http://git.php.net/?p=php-src.git;a=commit;h=a3e19527004dc67dfd875e758007b8ae135d4ed1
Log: Fixed bug #71153
 [2015-12-17 20:03 UTC] nikic@php.net
-Status: Assigned +Status: Closed
 [2016-07-20 11:34 UTC] davey@php.net
Automatic comment on behalf of nikic
Revision: http://git.php.net/?p=php-src.git;a=commit;h=a3e19527004dc67dfd875e758007b8ae135d4ed1
Log: Fixed bug #71153
 
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