Memory reclaim – a key kernel housekeeping task
As you will be aware, the kernel tries, for optimal performance, to keep the working set of memory pages as high up as possible in the memory pyramid (or hierarchy).
The so-called memory pyramid (or memory hierarchy) on a system consists of (in order, from smallest size but fastest speed to largest size but slowest speed): CPU registers, CPU caches (LI, L2, L3, ...), RAM, and swap (raw disk/flash/SSD partition). In the following discussion, we ignore CPU registers as their size is minuscule.
In a modern processor, as code executes and data is worked upon, the processor uses its hardware caches (L1, L2, and so on) to hold the current working set of pages within its multilevel CPU instruction and data caches. But of course, CPU cache memory is very limited, thus it will soon run out, causing the memory to spill over into the next hierarchical level – RAM. On modern systems, even many embedded ones, there...
