Master the php garbage collection mechanism

php's garbage collection mechanism can be simply summarized as reference counting, copy-on-write, and COW mechanism. This article mainly shares with you the knowledge of mastering the garbage collection mechanism of PHP, hoping to help everyone.

• Basic knowledge of reference counting The official website’s answer is as follows

• Each PHP variable exists in a variable container called "zval"

• A zval variable container, in addition to the type and value of the variable, also includes two bytes of additional information is_ref and refcount

• is_ref is a bool value used to identify whether this variable belongs to a reference set. Through this byte, the PHP engine can distinguish ordinary variables from reference variables

• refcount Used to represent the number of variables pointing to this zval variable container

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• Reference counting in PHP5

  In PHP5, zval memory is allocated separately from the heap (heap) (With a few exceptions), PHP needs to know which zvals are in use and which ones need to be freed. So this requires the use of reference counting: the value of refcount__gc in zval is used to save the number of times zval itself is referenced, such as

  b = 12 statement, 12 is referenced by two variables, so its reference count is 2. If the reference count becomes 0, it means that the variable is no longer used and the memory can be released.

  As follows

<?php

//php zval变量容器$a = 1;$b = 1;$c = &$a;$d = $b;$e = range(0, 3);

xdebug_debug_zval(&#39;a&#39;);
xdebug_debug_zval(&#39;b&#39;);
xdebug_debug_zval(&#39;c&#39;);
xdebug_debug_zval(&#39;d&#39;);
xdebug_debug_zval(&#39;e&#39;);

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

结果如下

a:(refcount=2, is_ref=1),int 1b:(refcount=2, is_ref=0),int 1c:(refcount=2, is_ref=1),int 1d:(refcount=2, is_ref=0),int 1e:(refcount=1, is_ref=0),
array (size=4)
  0 => (refcount=1, is_ref=0),int 0
  1 => (refcount=1, is_ref=0),int 1
  2 => (refcount=1, is_ref=0),int 2
  3 => (refcount=1, is_ref=0),int 3

Each variable has its own number

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• zval## in PHP7

  #In PHP7, zval has a new implementation. The most basic change is that the memory required by zval is no longer allocated separately from the heap, and

  no longer stores the reference count by itself. The reference count of complex data types (such as strings, arrays, and objects) is stored by itself. This implementation has the following benefits:

• Simple data types do not need to allocate memory separately, nor do they need to be counted

• No In the case of two counts, in the object, only the count stored in the object itself is valid

• Since the count is now stored by the value itself, it can be compared with the non-zval structure Data sharing, such as between zval and hashtable key

• The number of pointers required for indirect access is reduced

<?php

//php zval变量容器$a = 1;$b = 1;$c = &$a;$d = $b;$e = range(0, 3);

xdebug_debug_zval(&#39;a&#39;);
xdebug_debug_zval(&#39;b&#39;);
xdebug_debug_zval(&#39;c&#39;);
xdebug_debug_zval(&#39;d&#39;);
xdebug_debug_zval(&#39;e&#39;);


>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
结果如下

a:(refcount=2, is_ref=1)int 1b:(refcount=0, is_ref=0)int 1c:(refcount=2, is_ref=1)int 1d:(refcount=0, is_ref=0)int 1e:(refcount=1, is_ref=0)array (size=4)
  0 => (refcount=0, is_ref=0)int 0
  1 => (refcount=0, is_ref=0)int 1
  2 => (refcount=0, is_ref=0)int 2
  3 => (refcount=0, is_ref=0)int 3

Ordinary variables are no longer Remember your own number. Complex types like arrays remember their own numbers.

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• What is garbage
  Only under guideline 3, GC will collect zval up, and then use the new algorithm to determine whether this zval is garbage. So how to judge whether such a variable is real garbage?
  Simply put, it is to perform a refcount minus 1 operation for each element in this zval. After the operation is completed, if the refcount of zval=0, then this zval is garbage

• If the refcount of a zval increases, then the zval is still in use and is not garbage

##If the refcount of a zval decreases to 0, then the zval can be Released, not garbage
If the refcount of a zval is greater than 0 after being reduced, then the zval cannot be released, and the zval may become garbage
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