03-25-2014
Some supporting parts of the O/S run outside the kernel in processes, where they have a richer set of resources. The kernel wires down a certain amount of memory for code, i/o buffers and tables to support VM and i/o. The rest is shared via VM to all users, possibly even some functions of the kernel like disk cache. The methods for picking which page or RAM to assign to which page of what file or process address space vary (big bucks and deep thoughts). The hard rule is that dirty pages (written in RAM but not disk) must be written before they can be reassigned, and you do not want to erode the caller's working set, else the caller keeps calling for the same pages in rotation. They generally try to find the most idle pages but ensure all running processes have a reasonable 'working set' of pages in RAM. Sometimes high priority processes and pages get 'pin weights' that make them survive N more attempts at reassignment. Many pages are shared by many processes, so assigning ownership is difficult. Some O/S falsely mark pages unattached to see if another process will adopt them, but in a way that the page is not immediately freed up, so it can be readopted in place. Some processes can 'wire' some of their pages against the VM reassignment.
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