php session garbage collection mechanism

In PHP, when no variable points to this object, this object becomes garbage. PHP will destroy it in memory; this is PHP's GC garbage disposal mechanism to prevent memory overflow.

The job of GC is to scan all Session information, subtract the session last modified time from the current time, the same as the session.gc_maxlifetime parameter Compare, if the survival time exceeds gc_maxlifetime (default 24 minutes), the session will be deleted.

When a valid request occurs, PHP will decide whether to enable a GC based on the values ​​of global variables session.gc_probability and session.gc_divisor. By default In this case,

session.gc_probability = 1
session.gc_divisor = 100

That is to say, there is a 1% possibility of starting GC (that is, only one GC out of 100 requests will be started with one of the 100 requests).

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?

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

2. Increase the startup rate of GC. When the startup rate of GC increases, the performance of the system will decrease accordingly. It is not recommended;

3. Determine the current session in the code. 's survival time, use session_destroy()delete;

session.gc_probability = 1
session.gc_divisor = 100
session.gc_maxlifetime = 1440

These three configuration combinations build the garbage collection mechanism of the server-side session

session. gc_probability and session.gc_divisor constitute the probability of performing session cleaning. The theoretical explanation is that the server has a certain probability of regularly calling the gc function to clean the session. The probability of cleaning is: gc_probability/gc_divisor For example: 1/100 means that when each new session is initialized, there is a

1% probability that the garbage collection program will be started and cleaned. The standard is the time defined by session.gc_maxlifetime.

session.gc_divisor and session.gc_probability together define the gc (garbage collection garbage collection) process to be started when each session is initialized. The probability.

This probability is calculated using gc_probability/gc_divisor . For example 1/100 means there is a 1% probability of starting the gc process in each request. session.gc_divisor Defaults to 100.

For example:

session.gc_maxlifetime = 60
session.gc_divisor = 1000
session.gc_probability = 1

means that every time a thousand users call session_start(), the garbage collection mechanism will be executed 100%, and the garbage collection mechanism will be executed on the disk. Delete useless session files.

Note: Generally for some large portals, it is recommended to increase session.gc_divisor to reduce overhead

Next, I will demonstrate through an example how to Only by configuration can the gc (garbage collection) process be called!

By configuring the php.ini file, modify the following information:

session.gc_maxlifetime = 60//当session文件在60s后还没有被访问的话，则该session文件将会被视为“垃圾文件”，并且等待gc（垃圾回收）进程的调用的时候被清理掉
session.gc_probability = 1000

Because the probability of the gc process being called is calculated through gc_probability/gc_divisor, here I changed session.gc_probability to 1000, and session.gc_divisor is also 1000 by default. The gc process will be called every time the session_start() function is executed.

When three sessions are opened, three corresponding session files will be created. If each file is not called within 60 seconds, it will be regarded as a "junk file". When the gc process is called, the "junk file" will be unlink, because I have changed the probability of gc being called to 100% by modifying the php.ini configuration file. Hundreds, so next, if I use any browser to refresh the page, there should be only one of the three session files left

session.save_handler = files

The default is file, defining session In the server-side saving method, file means saving session to a temporary file. If we want to customize other ways of saving (such as using a database), we need to save this item Set to user.

Related recommendations: "PHP Tutorial"

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