在php.ini中可以看到以下配置行

; Define the probability that the 'garbage collection' process is started
; on every session initialization.
; The probability is calculated by using gc_probability/gc_pisor,
; e.g. 1/100 means there is a 1% chance that the GC process starts
; on each request.

session.gc_probability = 1
session.gc_pisor     = 100

; After this number of seconds, stored data will be seen as 'garbage' and
; cleaned up by the garbage collection process.
session.gc_maxlifetime = 1440

从注释可以看出是session垃圾回收的配置。 session.gc_probability/session.gc_pisor 表述有1%的概率回收大于session.gc_maxlifetime时间的过期文件。

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我们看 http://cn.php.net/manual/zh/sessionha... php文档对 session.gc 的说明

Cleans up expired sessions. Called randomly by PHP internally when a sessio starts or when session_start() is invoked. The frequency this is called is based on the session.gc_pisor and session.gc_probability configuration directives.

This method wraps the internal PHP save handler defined in the session.save_handler ini setting that was set before this handler was set by session_set_save_handler().

If this class is extended by inheritiance, calling the parent gc method will invoke the wrapper for this method and therefor invoke the associated internal callback. This allows this method to be overidden and or intercepted and filtered.

For more information on what this method is expected to do, please refer to the documentation at SessionHandlerInterface::gc().

即：比如session_start()一次会话请求，会有1%的概率调用session的垃圾回收来以清理过期会话。

session.gcprobability = 1
session.gcpisor = 100
session.gc_maxlifetime = 1440

这三个配置组合构建服务端session的垃圾回收机制

session.gc_probability与session.gc_pisor构成执行session清理的概率，理论上的解释为服务端定期有一定的概率调用gc函数来对session进行清理，清理的概率为：
gc_probability/gc_pisor 比如：1/100 表示每一个新会话初始化时，有1%的概率会启动垃圾回收程序，清理的标准为session.gc_maxlifetime定义的时间。

我写了一篇关于结合http分析php session 本质的文章，有兴趣的话可以看下 - 你必须了解的session的本质