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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 | |
b4fb3766 | 15 | a new memory policy via mbind(). The pages of process can also be relocated |
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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. | |
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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://ftp.suse.com/pub/people/ak). numactl provided libnuma which | |
22 | provides an interface similar to other numa functionality for page migration. | |
23 | cat /proc/<pid>/numa_maps allows an easy review of where the pages of | |
24 | a process are located. See also the numa_maps manpage in the numactl package. | |
25 | ||
26 | Manual migration is useful if for example the scheduler has relocated | |
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27 | a process to a processor on a distant node. A batch scheduler or an |
28 | administrator may detect the situation and move the pages of the process | |
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29 | nearer to the new processor. The kernel itself does only provide |
30 | manual page migration support. Automatic page migration may be implemented | |
31 | through user space processes that move pages. A special function call | |
32 | "move_pages" allows the moving of individual pages within a process. | |
33 | A NUMA profiler may f.e. obtain a log showing frequent off node | |
34 | accesses and may use the result to move pages to more advantageous | |
35 | locations. | |
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36 | |
37 | Larger installations usually partition the system using cpusets into | |
38 | sections of nodes. Paul Jackson has equipped cpusets with the ability to | |
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39 | move pages when a task is moved to another cpuset (See ../cpusets.txt). |
40 | Cpusets allows the automation of process locality. If a task is moved to | |
41 | a new cpuset then also all its pages are moved with it so that the | |
42 | performance of the process does not sink dramatically. Also the pages | |
43 | of processes in a cpuset are moved if the allowed memory nodes of a | |
44 | cpuset are changed. | |
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45 | |
46 | Page migration allows the preservation of the relative location of pages | |
47 | within a group of nodes for all migration techniques which will preserve a | |
48 | particular memory allocation pattern generated even after migrating a | |
49 | process. This is necessary in order to preserve the memory latencies. | |
50 | Processes will run with similar performance after migration. | |
51 | ||
52 | Page migration occurs in several steps. First a high level | |
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53 | description for those trying to use migrate_pages() from the kernel |
54 | (for userspace usage see the Andi Kleen's numactl package mentioned above) | |
55 | and then a low level description of how the low level details work. | |
a48d07af | 56 | |
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57 | A. In kernel use of migrate_pages() |
58 | ----------------------------------- | |
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59 | |
60 | 1. Remove pages from the LRU. | |
61 | ||
62 | Lists of pages to be migrated are generated by scanning over | |
63 | pages and moving them into lists. This is done by | |
b4fb3766 | 64 | calling isolate_lru_page(). |
a48d07af | 65 | Calling isolate_lru_page increases the references to the page |
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66 | so that it cannot vanish while the page migration occurs. |
67 | It also prevents the swapper or other scans to encounter | |
68 | the page. | |
a48d07af | 69 | |
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70 | 2. We need to have a function of type new_page_t that can be |
71 | passed to migrate_pages(). This function should figure out | |
72 | how to allocate the correct new page given the old page. | |
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73 | |
74 | 3. The migrate_pages() function is called which attempts | |
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75 | to do the migration. It will call the function to allocate |
76 | the new page for each page that is considered for | |
77 | moving. | |
a48d07af | 78 | |
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79 | B. How migrate_pages() works |
80 | ---------------------------- | |
a48d07af | 81 | |
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82 | migrate_pages() does several passes over its list of pages. A page is moved |
83 | if all references to a page are removable at the time. The page has | |
84 | already been removed from the LRU via isolate_lru_page() and the refcount | |
85 | is increased so that the page cannot be freed while page migration occurs. | |
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86 | |
87 | Steps: | |
88 | ||
89 | 1. Lock the page to be migrated | |
90 | ||
91 | 2. Insure that writeback is complete. | |
92 | ||
8d3c138b | 93 | 3. Prep the new page that we want to move to. It is locked |
a48d07af | 94 | and set to not being uptodate so that all accesses to the new |
b4fb3766 | 95 | page immediately lock while the move is in progress. |
a48d07af | 96 | |
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97 | 4. The new page is prepped with some settings from the old page so that |
98 | accesses to the new page will discover a page with the correct settings. | |
99 | ||
100 | 5. All the page table references to the page are converted | |
101 | to migration entries or dropped (nonlinear vmas). | |
102 | This decrease the mapcount of a page. If the resulting | |
103 | mapcount is not zero then we do not migrate the page. | |
104 | All user space processes that attempt to access the page | |
105 | will now wait on the page lock. | |
a48d07af | 106 | |
b4fb3766 | 107 | 6. The radix tree lock is taken. This will cause all processes trying |
8d3c138b | 108 | to access the page via the mapping to block on the radix tree spinlock. |
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109 | |
110 | 7. The refcount of the page is examined and we back out if references remain | |
111 | otherwise we know that we are the only one referencing this page. | |
112 | ||
113 | 8. The radix tree is checked and if it does not contain the pointer to this | |
8d3c138b | 114 | page then we back out because someone else modified the radix tree. |
a48d07af | 115 | |
8d3c138b | 116 | 9. The radix tree is changed to point to the new page. |
a48d07af | 117 | |
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118 | 10. The reference count of the old page is dropped because the radix tree |
119 | reference is gone. A reference to the new page is established because | |
120 | the new page is referenced to by the radix tree. | |
a48d07af | 121 | |
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122 | 11. The radix tree lock is dropped. With that lookups in the mapping |
123 | become possible again. Processes will move from spinning on the tree_lock | |
124 | to sleeping on the locked new page. | |
a48d07af | 125 | |
8d3c138b | 126 | 12. The page contents are copied to the new page. |
a48d07af | 127 | |
8d3c138b | 128 | 13. The remaining page flags are copied to the new page. |
a48d07af | 129 | |
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130 | 14. The old page flags are cleared to indicate that the page does |
131 | not provide any information anymore. | |
a48d07af | 132 | |
8d3c138b | 133 | 15. Queued up writeback on the new page is triggered. |
a48d07af | 134 | |
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135 | 16. If migration entries were page then replace them with real ptes. Doing |
136 | so will enable access for user space processes not already waiting for | |
137 | the page lock. | |
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138 | |
139 | 19. The page locks are dropped from the old and new page. | |
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140 | Processes waiting on the page lock will redo their page faults |
141 | and will reach the new page. | |
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142 | |
143 | 20. The new page is moved to the LRU and can be scanned by the swapper | |
144 | etc again. | |
145 | ||
8d3c138b | 146 | Christoph Lameter, May 8, 2006. |
a48d07af | 147 |