What is the php garbage collection mechanism?

PHP Garbage Collection Mechanism (Garbage Collector, GC for short) In PHP, when no variables point to this object, the object becomes garbage.

PHP will destroy it in memory; this is PHP's GC garbage disposal mechanism to prevent memory overflow. When a PHP thread ends, all memory space currently occupied will be destroyed, and all objects in the current program will be destroyed at the same time. (Recommended learning: PHP programming from entry to proficiency)

The GC process generally starts running with each SESSION. The purpose of .gc is to automatically destroy and delete the session files after they expire. File.

__destruct /unset __destruct The destructor is executed when the garbage object is recycled.

unset destroys the variable pointing to the object, not the object.

Session and PHP garbage collection mechanism Due to the working mechanism of PHP, it does not have a daemon thread to regularly scan Session information and determine whether it is invalid. When a valid request occurs, PHP will The values ​​of global variables session.gc_probability and session.gc_divisor determine whether to enable a GC.

By default, session.gc_probability=1, session.gc_divisor =100 means there is a 1% possibility of starting GC (that is, only one gc in 100 requests will be accompanied by 100 Started by one of the requests).

The job of the PHP garbage collection mechanism is to scan all Session information, subtract the last modification time of the session from the current time, and compare it with the session.gc_maxlifetime parameter. If it survives If the time exceeds gc_maxlifetime (default 24 minutes), the session will be deleted.

However, if your web server has multiple sites, GC may have unexpected results when processing sessions at multiple sites. The reason is: when GC is working, it will not distinguish between sessions of different sites.

So how to solve it at this time?

Modify session.save_path, or use session_save_path to save the session of each site to a dedicated directory,

Provide the startup rate of GC, and naturally, the startup rate of PHP garbage collection mechanism If it is increased, the system performance will be reduced accordingly, which is not recommended.

Judge the survival time of the current session in the code and use session_destroy to delete it.

Basic knowledge of reference counting

Each PHP variable exists in a variable container called "zval". A zval variable container, in addition to containing the type and value of the variable, It also includes two bytes of additional information.

The first one is "is_ref", which is a bool value used to identify whether this variable belongs to a reference set. Through this byte, the PHP engine To distinguish ordinary variables from reference variables. Since PHP allows users to use custom references by using &, there is also an internal reference counting mechanism in the zval variable container to optimize memory usage. The second extra byte is "refcount" , used to represent the number of variables (also called symbols) pointing to this zval variable container.

When a variable is assigned a constant value, a zval variable container will be generated, as shown in the following example:

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

In the above example, the new variable is a, which is generated in the current scope. And a variable container of type string and value "new string" is generated. In the extra two bytes In the information, "is_ref" is set to false by default because no custom reference is generated. "refcount" is set to 1 because there is only one variable using this variable container. Call xdebug to view the variable content:

<?php
$a = "new string";
xdebug_debug_zval(&#39;a&#39;);
?>

The above code will output:

a: (refcount=1, is_ref=0)='new string'

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