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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	Page migration
2	--------------
3	
4	Page migration allows the moving of the physical location of pages between
5	nodes in a numa system while the process is running. This means that the
6	virtual addresses that the process sees do not change. However, the
7	system rearranges the physical location of those pages.
8	
9	The main intend of page migration is to reduce the latency of memory access
10	by moving pages near to the processor where the process accessing that memory
11	is running.
12	
13	Page migration allows a process to manually relocate the node on which its
14	pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
15	a new memory policy via mbind(). The pages of process can also be relocated
16	from another process using the sys_migrate_pages() function call. The
17	migrate_pages function call takes two sets of nodes and moves pages of a
18	process that are located on the from nodes to the destination nodes.
19	Page migration functions are provided by the numactl package by Andi Kleen
20	(a version later than 0.9.3 is required. Get it from
21	ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma
22	which provides an interface similar to other numa functionality for page
23	migration.  cat /proc/<pid>/numa_maps allows an easy review of where the
24	pages of a process are located. See also the numa_maps documentation in the
25	proc(5) man page.
26	
27	Manual migration is useful if for example the scheduler has relocated
28	a process to a processor on a distant node. A batch scheduler or an
29	administrator may detect the situation and move the pages of the process
30	nearer to the new processor. The kernel itself does only provide
31	manual page migration support. Automatic page migration may be implemented
32	through user space processes that move pages. A special function call
33	"move_pages" allows the moving of individual pages within a process.
34	A NUMA profiler may f.e. obtain a log showing frequent off node
35	accesses and may use the result to move pages to more advantageous
36	locations.
37	
38	Larger installations usually partition the system using cpusets into
39	sections of nodes. Paul Jackson has equipped cpusets with the ability to
40	move pages when a task is moved to another cpuset (See
41	Documentation/cgroups/cpusets.txt).
42	Cpusets allows the automation of process locality. If a task is moved to
43	a new cpuset then also all its pages are moved with it so that the
44	performance of the process does not sink dramatically. Also the pages
45	of processes in a cpuset are moved if the allowed memory nodes of a
46	cpuset are changed.
47	
48	Page migration allows the preservation of the relative location of pages
49	within a group of nodes for all migration techniques which will preserve a
50	particular memory allocation pattern generated even after migrating a
51	process. This is necessary in order to preserve the memory latencies.
52	Processes will run with similar performance after migration.
53	
54	Page migration occurs in several steps. First a high level
55	description for those trying to use migrate_pages() from the kernel
56	(for userspace usage see the Andi Kleen's numactl package mentioned above)
57	and then a low level description of how the low level details work.
58	
59	A. In kernel use of migrate_pages()
60	-----------------------------------
61	
62	1. Remove pages from the LRU.
63	
64	   Lists of pages to be migrated are generated by scanning over
65	   pages and moving them into lists. This is done by
66	   calling isolate_lru_page().
67	   Calling isolate_lru_page increases the references to the page
68	   so that it cannot vanish while the page migration occurs.
69	   It also prevents the swapper or other scans to encounter
70	   the page.
71	
72	2. We need to have a function of type new_page_t that can be
73	   passed to migrate_pages(). This function should figure out
74	   how to allocate the correct new page given the old page.
75	
76	3. The migrate_pages() function is called which attempts
77	   to do the migration. It will call the function to allocate
78	   the new page for each page that is considered for
79	   moving.
80	
81	B. How migrate_pages() works
82	----------------------------
83	
84	migrate_pages() does several passes over its list of pages. A page is moved
85	if all references to a page are removable at the time. The page has
86	already been removed from the LRU via isolate_lru_page() and the refcount
87	is increased so that the page cannot be freed while page migration occurs.
88	
89	Steps:
90	
91	1. Lock the page to be migrated
92	
93	2. Insure that writeback is complete.
94	
95	3. Prep the new page that we want to move to. It is locked
96	   and set to not being uptodate so that all accesses to the new
97	   page immediately lock while the move is in progress.
98	
99	4. The new page is prepped with some settings from the old page so that
100	   accesses to the new page will discover a page with the correct settings.
101	
102	5. All the page table references to the page are converted
103	   to migration entries or dropped (nonlinear vmas).
104	   This decrease the mapcount of a page. If the resulting
105	   mapcount is not zero then we do not migrate the page.
106	   All user space processes that attempt to access the page
107	   will now wait on the page lock.
108	
109	6. The radix tree lock is taken. This will cause all processes trying
110	   to access the page via the mapping to block on the radix tree spinlock.
111	
112	7. The refcount of the page is examined and we back out if references remain
113	   otherwise we know that we are the only one referencing this page.
114	
115	8. The radix tree is checked and if it does not contain the pointer to this
116	   page then we back out because someone else modified the radix tree.
117	
118	9. The radix tree is changed to point to the new page.
119	
120	10. The reference count of the old page is dropped because the radix tree
121	    reference is gone. A reference to the new page is established because
122	    the new page is referenced to by the radix tree.
123	
124	11. The radix tree lock is dropped. With that lookups in the mapping
125	    become possible again. Processes will move from spinning on the tree_lock
126	    to sleeping on the locked new page.
127	
128	12. The page contents are copied to the new page.
129	
130	13. The remaining page flags are copied to the new page.
131	
132	14. The old page flags are cleared to indicate that the page does
133	    not provide any information anymore.
134	
135	15. Queued up writeback on the new page is triggered.
136	
137	16. If migration entries were page then replace them with real ptes. Doing
138	    so will enable access for user space processes not already waiting for
139	    the page lock.
140	
141	19. The page locks are dropped from the old and new page.
142	    Processes waiting on the page lock will redo their page faults
143	    and will reach the new page.
144	
145	20. The new page is moved to the LRU and can be scanned by the swapper
146	    etc again.
147	
148	Christoph Lameter, May 8, 2006.
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