Explain the PHP garbage collection mechanism in detail through examples

PHP garbage collection mechanism:

1. PHP can automatically manage memory and clear unnecessary objects, mainly using reference counting

2 . Ref_count and is_ref are defined in the zval structure. ref_count is a reference count, which identifies how many variables this zval is referenced. When it is 0, it will be destroyed. is_ref identifies whether the & address character is used to force a reference

3. In order to solve the problem of circular reference memory leaks, a synchronous cycle recycling algorithm is used.

For example, when an array or object cyclically refers to itself and unsets the array, if refcount-1 is still greater than 0, it will be regarded as suspected garbage, traversed, and simulated deletion of refcount- 1 If it is 0, delete it. If it is not 0, restore the stubborn garbage generation process:

<?php
    $a = "new string";
?>

a: (refcount_gc=1, is_ref_gc=0)='new string'

When $a is assigned to another variable, the refcount_gc of the zval corresponding to $a will be increased by 1

<?php
    $a = "new string";
    $b = $a;
?>

At this time, the internal storage information corresponding to the $a and $b variables is, $a and $b point to a string "new string" at the same time, and its refcount becomes 2a,b: (refcount_gc=2,is_ref= 0)='new string'

When unset is used to delete the $b variable, the refcount_gc of "new string" will be reduced by 1 and become 1

For ordinary variables, all this It's normal, but in composite type variables (arrays and objects), something more interesting will happen:

<?php
    $a = array(&#39;meaning&#39; => &#39;life&#39;, &#39;number&#39; => 42);
?>

$aThe internal storage information is:

a: (refcount=1, is_ref=0)=array (
'meaning' => (refcount=1, is_ref=0)='life',
'number' => (refcount=1, is_ref=0)=42
)

The array variable itself ($a) Inside the engine is actually a hash table. There are two zval items meaning and number in this table, so in fact a total of 3 zvals are generated in that line of code. These 3 zvals all follow the reference and counting principles of variables. Use The diagram shows:

# Next, add an element to $a and assign the value of an existing element to the new element:

<?php
    $a = array(&#39;meaning&#39; => &#39;life&#39;, &#39;number&#39; => 42);
    $a[&#39;name&#39;] = $a[&#39;meaning&#39;];
 ?>

Then the internal storage of $a is, the ref_count of "life" becomes 2, and the ref_count of 42 is 1:

a: (refcount=1, is_ref=0)=array (
'meaning' => (refcount=2, is_ref=0)='life',
'number' => (refcount=1, is_ref=0)=42,
'name' => (refcount=2, is_ref=0)='life'
)

If you assign the reference of the array to an element in the array, interesting things will happen:

<?php
    $a = array(&#39;one&#39;);
    $a[] = &$a;
?>

In this way, the $a array has two elements, one with an index of 0 and a value of the character one, and the other with an index of 1, which is a reference to $a itself. The internal storage is as follows:

a: (refcount=2, is_ref=1)=array (
0 => (refcount=1, is_ref=0)='one',
1 => (refcount=2, is_ref=1)=…
)

array The ref_count of this zval is 2, which is a circular reference. At this time, $a is unset, then $a will be deleted from the symbol table, and the refcount_gc of the zval pointed to by $a is reduced by 1.

Then the problem arises, $a is no longer in the symbol table, the user This variable can no longer be accessed, but the refcount_gc of the zval pointed to by $a becomes 1 instead of 0, so it cannot be recycled, resulting in a memory leak. The job of the new GC is to clean up such garbage.

In order to solve the problem of circular reference memory leaks, a synchronous cycle recycling algorithm is used. If the ref_count is reduced by 1 and is still greater than 0, it will be regarded as suspected garbage.

For example, when an array or object cyclically refers to itself and unsets the array, if refcount-1 is still greater than 0, it will be traversed and simulated to delete refcount-1 once. If it is 0, it will be deleted. If it is not 0, restore it.

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