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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 | |
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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. | |
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26 | |
27 | Manual migration is useful if for example the scheduler has relocated | |
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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 | |
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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. | |
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37 | |
38 | Larger installations usually partition the system using cpusets into | |
39 | sections of nodes. Paul Jackson has equipped cpusets with the ability to | |
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40 | move pages when a task is moved to another cpuset (See |
41 | Documentation/cgroups/cpusets.txt). | |
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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. | |
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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 | |
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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. | |
a48d07af | 58 | |
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59 | A. In kernel use of migrate_pages() |
60 | ----------------------------------- | |
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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 | |
b4fb3766 | 66 | calling isolate_lru_page(). |
a48d07af | 67 | Calling isolate_lru_page increases the references to the page |
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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. | |
a48d07af | 71 | |
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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. | |
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75 | |
76 | 3. The migrate_pages() function is called which attempts | |
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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. | |
a48d07af | 80 | |
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81 | B. How migrate_pages() works |
82 | ---------------------------- | |
a48d07af | 83 | |
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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. | |
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88 | |
89 | Steps: | |
90 | ||
91 | 1. Lock the page to be migrated | |
92 | ||
93 | 2. Insure that writeback is complete. | |
94 | ||
8d3c138b | 95 | 3. Prep the new page that we want to move to. It is locked |
a48d07af | 96 | and set to not being uptodate so that all accesses to the new |
b4fb3766 | 97 | page immediately lock while the move is in progress. |
a48d07af | 98 | |
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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. | |
a48d07af | 108 | |
b4fb3766 | 109 | 6. The radix tree lock is taken. This will cause all processes trying |
8d3c138b | 110 | to access the page via the mapping to block on the radix tree spinlock. |
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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 | |
8d3c138b | 116 | page then we back out because someone else modified the radix tree. |
a48d07af | 117 | |
8d3c138b | 118 | 9. The radix tree is changed to point to the new page. |
a48d07af | 119 | |
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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. | |
a48d07af | 123 | |
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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. | |
a48d07af | 127 | |
8d3c138b | 128 | 12. The page contents are copied to the new page. |
a48d07af | 129 | |
8d3c138b | 130 | 13. The remaining page flags are copied to the new page. |
a48d07af | 131 | |
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132 | 14. The old page flags are cleared to indicate that the page does |
133 | not provide any information anymore. | |
a48d07af | 134 | |
8d3c138b | 135 | 15. Queued up writeback on the new page is triggered. |
a48d07af | 136 | |
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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. | |
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140 | |
141 | 19. The page locks are dropped from the old and new page. | |
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142 | Processes waiting on the page lock will redo their page faults |
143 | and will reach the new page. | |
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144 | |
145 | 20. The new page is moved to the LRU and can be scanned by the swapper | |
146 | etc again. | |
147 | ||
8d3c138b | 148 | Christoph Lameter, May 8, 2006. |
a48d07af | 149 |