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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 | |
29 | nearer to the new processor. At some point in the future we may have | |
30 | some mechanism in the scheduler that will automatically move the pages. | |
31 | ||
32 | Larger installations usually partition the system using cpusets into | |
33 | sections of nodes. Paul Jackson has equipped cpusets with the ability to | |
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34 | move pages when a task is moved to another cpuset (See ../cpusets.txt). |
35 | Cpusets allows the automation of process locality. If a task is moved to | |
36 | a new cpuset then also all its pages are moved with it so that the | |
37 | performance of the process does not sink dramatically. Also the pages | |
38 | of processes in a cpuset are moved if the allowed memory nodes of a | |
39 | cpuset are changed. | |
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40 | |
41 | Page migration allows the preservation of the relative location of pages | |
42 | within a group of nodes for all migration techniques which will preserve a | |
43 | particular memory allocation pattern generated even after migrating a | |
44 | process. This is necessary in order to preserve the memory latencies. | |
45 | Processes will run with similar performance after migration. | |
46 | ||
47 | Page migration occurs in several steps. First a high level | |
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48 | description for those trying to use migrate_pages() from the kernel |
49 | (for userspace usage see the Andi Kleen's numactl package mentioned above) | |
50 | and then a low level description of how the low level details work. | |
a48d07af | 51 | |
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52 | A. In kernel use of migrate_pages() |
53 | ----------------------------------- | |
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54 | |
55 | 1. Remove pages from the LRU. | |
56 | ||
57 | Lists of pages to be migrated are generated by scanning over | |
58 | pages and moving them into lists. This is done by | |
b4fb3766 | 59 | calling isolate_lru_page(). |
a48d07af | 60 | Calling isolate_lru_page increases the references to the page |
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61 | so that it cannot vanish while the page migration occurs. |
62 | It also prevents the swapper or other scans to encounter | |
63 | the page. | |
a48d07af | 64 | |
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65 | 2. Generate a list of newly allocates page. These pages will contain the |
66 | contents of the pages from the first list after page migration is | |
67 | complete. | |
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68 | |
69 | 3. The migrate_pages() function is called which attempts | |
70 | to do the migration. It returns the moved pages in the | |
71 | list specified as the third parameter and the failed | |
72 | migrations in the fourth parameter. The first parameter | |
73 | will contain the pages that could still be retried. | |
74 | ||
75 | 4. The leftover pages of various types are returned | |
76 | to the LRU using putback_to_lru_pages() or otherwise | |
77 | disposed of. The pages will still have the refcount as | |
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78 | increased by isolate_lru_pages() if putback_to_lru_pages() is not |
79 | used! The kernel may want to handle the various cases of failures in | |
80 | different ways. | |
a48d07af | 81 | |
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82 | B. How migrate_pages() works |
83 | ---------------------------- | |
a48d07af | 84 | |
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85 | migrate_pages() does several passes over its list of pages. A page is moved |
86 | if all references to a page are removable at the time. The page has | |
87 | already been removed from the LRU via isolate_lru_page() and the refcount | |
88 | is increased so that the page cannot be freed while page migration occurs. | |
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89 | |
90 | Steps: | |
91 | ||
92 | 1. Lock the page to be migrated | |
93 | ||
94 | 2. Insure that writeback is complete. | |
95 | ||
96 | 3. Make sure that the page has assigned swap cache entry if | |
97 | it is an anonyous page. The swap cache reference is necessary | |
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98 | to preserve the information contain in the page table maps while |
99 | page migration occurs. | |
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100 | |
101 | 4. Prep the new page that we want to move to. It is locked | |
102 | and set to not being uptodate so that all accesses to the new | |
b4fb3766 | 103 | page immediately lock while the move is in progress. |
a48d07af | 104 | |
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105 | 5. All the page table references to the page are either dropped (file |
106 | backed pages) or converted to swap references (anonymous pages). | |
107 | This should decrease the reference count. | |
a48d07af | 108 | |
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109 | 6. The radix tree lock is taken. This will cause all processes trying |
110 | to reestablish a pte 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 | |
b4fb3766 | 116 | page then we back out because someone else modified the mapping first. |
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117 | |
118 | 9. The mapping is checked. If the mapping is gone then a truncate action may | |
119 | be in progress and we back out. | |
120 | ||
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121 | 10. The new page is prepped with some settings from the old page so that |
122 | accesses to the new page will be discovered to have the correct settings. | |
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123 | |
124 | 11. The radix tree is changed to point to the new page. | |
125 | ||
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126 | 12. The reference count of the old page is dropped because the radix tree |
127 | reference is gone. | |
a48d07af | 128 | |
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129 | 13. The radix tree lock is dropped. With that lookups become possible again |
130 | and other processes will move from spinning on the tree lock to sleeping on | |
131 | the locked new page. | |
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132 | |
133 | 14. The page contents are copied to the new page. | |
134 | ||
135 | 15. The remaining page flags are copied to the new page. | |
136 | ||
137 | 16. The old page flags are cleared to indicate that the page does | |
138 | not use any information anymore. | |
139 | ||
140 | 17. Queued up writeback on the new page is triggered. | |
141 | ||
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142 | 18. If swap pte's were generated for the page then replace them with real |
143 | ptes. This will reenable access for processes not blocked by the page lock. | |
144 | ||
145 | 19. The page locks are dropped from the old and new page. | |
146 | Processes waiting on the page lock can continue. | |
147 | ||
148 | 20. The new page is moved to the LRU and can be scanned by the swapper | |
149 | etc again. | |
150 | ||
151 | TODO list | |
152 | --------- | |
153 | ||
154 | - Page migration requires the use of swap handles to preserve the | |
155 | information of the anonymous page table entries. This means that swap | |
156 | space is reserved but never used. The maximum number of swap handles used | |
157 | is determined by CHUNK_SIZE (see mm/mempolicy.c) per ongoing migration. | |
158 | Reservation of pages could be avoided by having a special type of swap | |
159 | handle that does not require swap space and that would only track the page | |
160 | references. Something like that was proposed by Marcelo Tosatti in the | |
161 | past (search for migration cache on lkml or linux-mm@kvack.org). | |
a48d07af | 162 | |
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163 | - Page migration unmaps ptes for file backed pages and requires page |
164 | faults to reestablish these ptes. This could be optimized by somehow | |
165 | recording the references before migration and then reestablish them later. | |
166 | However, there are several locking challenges that have to be overcome | |
167 | before this is possible. | |
a48d07af | 168 | |
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169 | - Page migration generates read ptes for anonymous pages. Dirty page |
170 | faults are required to make the pages writable again. It may be possible | |
171 | to generate a pte marked dirty if it is known that the page is dirty and | |
172 | that this process has the only reference to that page. | |
a48d07af | 173 | |
b4fb3766 | 174 | Christoph Lameter, March 8, 2006. |
a48d07af | 175 |