Caches are good for performance. Visualize reading the content of a large file from disk as opposed to reading its content from RAM. There's no question that the RAM-based I/O is much faster! As can be imagined, the Linux kernel leverages these ideas and thus maintains several caches – the page cache, dentry cache, inode cache, slab caches, and so on. These caches indeed greatly help performance, but, thinking about it, are not actually a mandatory requirement. When memory pressure reaches high levels (implying that too much memory is in use and too little is free), the Linux kernel has mechanisms to intelligently free up caches (aka memory reclamation - it's a continuous ongoing process; kernel threads (typically named kswapd*) reclaim memory as part of their housekeeping chores; more on this in the Reclaiming memory – a kernel housekeeping task and OOM section).
In the case of the slab cache(s), the fact is that some kernel...
