Insufficient system memory: When the system memory is low, Linux may reserve some cache to improve performance. If the system needs more memory to run new applications, the cache may not be released.
Applications running in the foreground may need to cache data: if some foreground-running applications frequently access certain data, Linux may keep cache of these data to improve access speed.
System settings: Some system settings may cause Linux to not release cache. For example, adjusting the vm.min_free_kbytes parameter can control the threshold at which the system releases memory.
Optimizing hard drive I/O performance: Linux may use caching to enhance the read and write performance of the hard drive. Releasing the cache could cause a decrease in hard drive I/O performance, so the system may retain the cache.
Linux uses file system caching to speed up file read and write operations. Releasing the file system cache can result in performance degradation, so the system may retain the cache.
