When the memory cache region becomes full, LRU removes the leastrecentlyused cached data first.
当内存缓存区域满时，lru会首先删除最近最少使用的缓存数据。
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Entries for the dentry cache are allocated from the dentry_cache slab allocator and use a least-recently-used (LRU) algorithm to prune entries when memory pressure exists.
Many servlet containers place a limit on the amount of memory that can be used across all sessions, and when that limit is reached, will serialize the leastrecently used session and write it to disk.