1Page migration 2-------------- 3 4Page migration allows the moving of the physical location of pages between 5nodes in a numa system while the process is running. This means that the 6virtual addresses that the process sees do not change. However, the 7system rearranges the physical location of those pages. 8 9The main intend of page migration is to reduce the latency of memory access 10by moving pages near to the processor where the process accessing that memory 11is running. 12 13Page migration allows a process to manually relocate the node on which its 14pages are located through the MF_MOVE and MF_MOVE_ALL options while setting 15a new memory policy via mbind(). The pages of process can also be relocated 16from another process using the sys_migrate_pages() function call. The 17migrate_pages function call takes two sets of nodes and moves pages of a 18process that are located on the from nodes to the destination nodes. 19Page migration functions are provided by the numactl package by Andi Kleen 20(a version later than 0.9.3 is required. Get it from 21ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma 22which provides an interface similar to other numa functionality for page 23migration. cat /proc/<pid>/numa_maps allows an easy review of where the 24pages of a process are located. See also the numa_maps documentation in the 25proc(5) man page. 26 27Manual migration is useful if for example the scheduler has relocated 28a process to a processor on a distant node. A batch scheduler or an 29administrator may detect the situation and move the pages of the process 30nearer to the new processor. The kernel itself does only provide 31manual page migration support. Automatic page migration may be implemented 32through user space processes that move pages. A special function call 33"move_pages" allows the moving of individual pages within a process. 34A NUMA profiler may f.e. obtain a log showing frequent off node 35accesses and may use the result to move pages to more advantageous 36locations. 37 38Larger installations usually partition the system using cpusets into 39sections of nodes. Paul Jackson has equipped cpusets with the ability to 40move pages when a task is moved to another cpuset (See ../cpusets.txt). 41Cpusets allows the automation of process locality. If a task is moved to 42a new cpuset then also all its pages are moved with it so that the 43performance of the process does not sink dramatically. Also the pages 44of processes in a cpuset are moved if the allowed memory nodes of a 45cpuset are changed. 46 47Page migration allows the preservation of the relative location of pages 48within a group of nodes for all migration techniques which will preserve a 49particular memory allocation pattern generated even after migrating a 50process. This is necessary in order to preserve the memory latencies. 51Processes will run with similar performance after migration. 52 53Page migration occurs in several steps. First a high level 54description for those trying to use migrate_pages() from the kernel 55(for userspace usage see the Andi Kleen's numactl package mentioned above) 56and then a low level description of how the low level details work. 57 58A. In kernel use of migrate_pages() 59----------------------------------- 60 611. Remove pages from the LRU. 62 63 Lists of pages to be migrated are generated by scanning over 64 pages and moving them into lists. This is done by 65 calling isolate_lru_page(). 66 Calling isolate_lru_page increases the references to the page 67 so that it cannot vanish while the page migration occurs. 68 It also prevents the swapper or other scans to encounter 69 the page. 70 712. We need to have a function of type new_page_t that can be 72 passed to migrate_pages(). This function should figure out 73 how to allocate the correct new page given the old page. 74 753. The migrate_pages() function is called which attempts 76 to do the migration. It will call the function to allocate 77 the new page for each page that is considered for 78 moving. 79 80B. How migrate_pages() works 81---------------------------- 82 83migrate_pages() does several passes over its list of pages. A page is moved 84if all references to a page are removable at the time. The page has 85already been removed from the LRU via isolate_lru_page() and the refcount 86is increased so that the page cannot be freed while page migration occurs. 87 88Steps: 89 901. Lock the page to be migrated 91 922. Insure that writeback is complete. 93 943. Prep the new page that we want to move to. It is locked 95 and set to not being uptodate so that all accesses to the new 96 page immediately lock while the move is in progress. 97 984. The new page is prepped with some settings from the old page so that 99 accesses to the new page will discover a page with the correct settings. 100 1015. All the page table references to the page are converted 102 to migration entries or dropped (nonlinear vmas). 103 This decrease the mapcount of a page. If the resulting 104 mapcount is not zero then we do not migrate the page. 105 All user space processes that attempt to access the page 106 will now wait on the page lock. 107 1086. The radix tree lock is taken. This will cause all processes trying 109 to access the page via the mapping to block on the radix tree spinlock. 110 1117. The refcount of the page is examined and we back out if references remain 112 otherwise we know that we are the only one referencing this page. 113 1148. The radix tree is checked and if it does not contain the pointer to this 115 page then we back out because someone else modified the radix tree. 116 1179. The radix tree is changed to point to the new page. 118 11910. The reference count of the old page is dropped because the radix tree 120 reference is gone. A reference to the new page is established because 121 the new page is referenced to by the radix tree. 122 12311. The radix tree lock is dropped. With that lookups in the mapping 124 become possible again. Processes will move from spinning on the tree_lock 125 to sleeping on the locked new page. 126 12712. The page contents are copied to the new page. 128 12913. The remaining page flags are copied to the new page. 130 13114. The old page flags are cleared to indicate that the page does 132 not provide any information anymore. 133 13415. Queued up writeback on the new page is triggered. 135 13616. If migration entries were page then replace them with real ptes. Doing 137 so will enable access for user space processes not already waiting for 138 the page lock. 139 14019. The page locks are dropped from the old and new page. 141 Processes waiting on the page lock will redo their page faults 142 and will reach the new page. 143 14420. The new page is moved to the LRU and can be scanned by the swapper 145 etc again. 146 147Christoph Lameter, May 8, 2006. 148 149