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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. |
149 | |
150 |
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