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Operating Systems Linux How does the Operating System handle memory? Post 302894491 by DGPickett on Tuesday 25th of March 2014 05:13:30 PM
Old 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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MIGRATEPAGES(8) 					   Linux Administrator's Manual 					   MIGRATEPAGES(8)

NAME
migratepages - Migrate the physical location a processes pages SYNOPSIS
migratepages pid from-nodes to-nodes DESCRIPTION
migratepages moves the physical location of a processes pages without any changes of the virtual address space of the process. Moving the pages allows one to change the distances of a process to its memory. Performance may be optimized by moving a processes pages to the node where it is executing. If multiple nodes are specified for from-nodes or to-nodes then an attempt is made to preserve the relative location of each page in each nodeset. For example if we move from nodes 2-5 to 7,9,12-13 then the preferred mode of operation is to move pages from 2->7, 3->9, 4->12 and 5->13. However, this is only posssible if enough memory is available. Valid node specifiers all All nodes number Node number number1{,number2} Node number1 and Node number2 number1-number2 Nodes from number1 to number2 ! nodes Invert selection of the following specification. NOTES
Requires an NUMA policy aware kernel with support for page migration (linux 2.6.16 and later). migratepages will only move pages that are not shared with other processes if called by a user without administrative priviledges (but with the right to modify the process). migratepages will move all pages if invoked from root (or a user with administrative priviledges). FILES
/proc/<pid>/numa_maps for information about the NUMA memory use of a process. COPYRIGHT
Copyright 2005-2006 Christoph Lameter, Silicon Graphics, Inc. migratepages is under the GNU General Public License, v.2 SEE ALSO
numactl(8) , set_mempolicy(2) , get_mempolicy(2) , mbind(2) , sched_setaffinity(2) , sched_getaffinity(2) , proc(5) , ftok(3) , shmat(2) , taskset(1) SGI
Jan 2005 MIGRATEPAGES(8)
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