Redis提供高效过期回收机制（redis过期回收线程）

Redis is a kind of memory cache and it is quite popular around the world. A basic feature of Redis is that it supports a high-efficiency expiration mechanism, which provides intelligent ability to recycle expired data. Here are the detls of how Redis implements the mechanism:

1. Redis employs the concept of a “virtual time,” which mntns a timestamp that dictates when certn entries expire. Whenever an item is added to the cache, the expiration time is recorded, and the virtual time is updated accordingly.

2. The expiration mechanism is implemented as a background task that runs regularly. When running, the task scans through the items in the cache to locate expired items, and then removes them from the cache.

3. Redis also supports setting an expiration time that allows for more efficient expiration logic. Specifically, if two entry’s expiration times are more than two weeks apart, only the newest expiration time is stored. The goal of this approach is to minimize the amount of metadata that needs to be managed.

4. Finally, Redis provides an efficient way to update or remove expiration times. Once an expiration time is stored in the cache, a special set of commands is used to update or remove the expiration time.

In this way, Redis provides a powerful expiration mechanism that ensures that only valid entries are stored in the cache. Moreover, its efficient implementation of the mechanism helps to reduce the amount of metadata that needs to be managed.