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 allSessioninformation, subtract thesessionlast modified time from the current time, the same as thesession.gc_maxlifetimeparameter Compare, if the survival time exceedsgc_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 variablessession.gc_probabilityandsession.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. Modifysession.save_path, or usesession_save_path()to save thesessionof 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 currentsession in the code.'s survival time, usesession_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_probabilityandsession.gc_divisorconstitute 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_divisorFor example:1/100means 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 bysession.gc_maxlifetime.

session.gc_divisorandsession.gc_probabilitytogether define thegc (garbage collection garbage collection)process to be started when each session is initialized. The probability.

This probability is calculated usinggc_probability/gc_divisor. For example 1/100 means there is a1%probability of starting the gc process in each request.session.gc_divisorDefaults to100.

For example:

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

means that every time a thousand users callsession_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 increasesession.gc_divisorto 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 throughgc_probability/gc_divisor, here I changedsession.gc_probabilityto1000, and session.gc_divisor is also1000by default. The gc process will be called every time the session_start() function is executed.

When three sessions are opened, three correspondingsessionfiles 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 beunlink, because I have changed the probability of gc being called to 100% by modifying thephp.iniconfiguration file. Hundreds, so next, if I use any browser to refresh the page, there should be only one of the threesessionfiles left

session.save_handler = files

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

Related recommendations: "PHP Tutorial"

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