Maintaining an ample supply of free pages for the system is a key duty of the kernel; to this end, the kernel symbol freemem contains the total number of page frames currently on a free list. When memory pressure is high, there are relatively few free pages on the free lists, and the kernel must take corrective action. Figure 7-1 gives us an overall view of the free-page monitoring system.
The total amount of memory available to the operating system is determined at boot and stored in the kernel parameter phys_mem_pages. Additional setpoints are calculated and continuously monitored by the kernel.