メモリ管理とガベージコレクション

PHPのメモリ管理

zval

<?php
// debug_zval_refcount-style inspection via gc + count
$arr = [1, 2, 3];
$copy = $arr;   // refcount of the array bumps, no data copy yet (CoW)

// Modifying triggers copy-on-write separation
$copy[] = 4;

var_dump($arr);   // still [1,2,3]
var_dump($copy);  // [1,2,3,4]
?>

参照カウント

<?php
$a = str_repeat('x', 1000);  // refcount 1 on the string
$b = $a;                      // refcount 2
unset($a);                    // refcount 1
unset($b);                    // refcount 0 -> freed instantly

echo "Refcounting frees deterministically\n";
?>

コピーオンライト

<?php
$big = range(1, 100000);
$shared = $big;          // O(1): shared, refcount++

// Reads stay shared and cheap
echo count($shared), PHP_EOL;

// First write separates (copies) — now it costs O(n)
$shared[0] = -1;
echo $big[0], ' vs ', $shared[0], PHP_EOL;  // 1 vs -1
?>

問題：循環参照

<?php
class Node { public ?Node $ref = null; }

$a = new Node();
$b = new Node();
$a->ref = $b;   // a -> b
$b->ref = $a;   // b -> a  (cycle!)

unset($a, $b);  // external refs gone, but internal cycle keeps refcount > 0

// The objects are unreachable but NOT yet freed by refcounting alone.
echo gc_collect_cycles() . " cycles collected\n";
?>

循環参照コレクタ

<?php
// GC status exposes the root buffer and run stats
for ($i = 0; $i < 3; $i++) {
    $a = new stdClass();
    $b = new stdClass();
    $a->b = $b; $b->a = $a;   // create a cycle
    unset($a, $b);            // becomes a GC root
}

print_r(gc_status());
?>

GCの制御

<?php
gc_disable();              // no automatic cycle collection

// ... heavy phase that creates and drops many cycles ...
for ($i = 0; $i < 1000; $i++) {
    $x = new stdClass(); $y = new stdClass();
    $x->y = $y; $y->x = $x; unset($x, $y);
}

$freed = gc_collect_cycles();   // reclaim all at once
gc_enable();
echo "Freed $freed objects\n";
?>

長時間稼働するワーカー

<?php
$peakStart = memory_get_usage(true);

// Simulate processing a batch of jobs
for ($job = 0; $job < 5; $job++) {
    $payload = range(0, 10000);
    // ... process ...
    unset($payload);
}

printf("start: %d KB, peak: %d KB, now: %d KB\n",
    $peakStart >> 10,
    memory_get_peak_usage(true) >> 10,
    memory_get_usage(true) >> 10
);
?>

WeakReferenceとWeakMap

<?php
$map = new WeakMap();

$obj = new stdClass();
$map[$obj] = 'metadata';      // does NOT keep $obj alive

echo count($map) . PHP_EOL;   // 1
unset($obj);                  // object collectible -> entry removed
echo count($map) . PHP_EOL;   // 0
?>

インターン文字列と不変データ

メモリ制限とアロケータ

<?php
echo 'limit: ' . ini_get('memory_limit') . PHP_EOL;
echo 'emalloc usage: ' . memory_get_usage() . " bytes\n";   // ZendMM tracked
echo 'real usage:    ' . memory_get_usage(true) . " bytes\n"; // from OS
?>

確認問題

まとめ