linux/Documentation/vm/page_migration
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   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
  41Documentation/cgroups/cpusets.txt).
  42Cpusets allows the automation of process locality. If a task is moved to
  43a new cpuset then also all its pages are moved with it so that the
  44performance of the process does not sink dramatically. Also the pages
  45of processes in a cpuset are moved if the allowed memory nodes of a
  46cpuset are changed.
  47
  48Page migration allows the preservation of the relative location of pages
  49within a group of nodes for all migration techniques which will preserve a
  50particular memory allocation pattern generated even after migrating a
  51process. This is necessary in order to preserve the memory latencies.
  52Processes will run with similar performance after migration.
  53
  54Page migration occurs in several steps. First a high level
  55description for those trying to use migrate_pages() from the kernel
  56(for userspace usage see the Andi Kleen's numactl package mentioned above)
  57and then a low level description of how the low level details work.
  58
  59A. In kernel use of migrate_pages()
  60-----------------------------------
  61
  621. 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
  722. 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
  763. 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
  81B. How migrate_pages() works
  82----------------------------
  83
  84migrate_pages() does several passes over its list of pages. A page is moved
  85if all references to a page are removable at the time. The page has
  86already been removed from the LRU via isolate_lru_page() and the refcount
  87is increased so that the page cannot be freed while page migration occurs.
  88
  89Steps:
  90
  911. Lock the page to be migrated
  92
  932. Insure that writeback is complete.
  94
  953. 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
  994. 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
 1025. 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
 1096. 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
 1127. 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
 1158. 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
 1189. The radix tree is changed to point to the new page.
 119
 12010. 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
 12411. 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
 12812. The page contents are copied to the new page.
 129
 13013. The remaining page flags are copied to the new page.
 131
 13214. The old page flags are cleared to indicate that the page does
 133    not provide any information anymore.
 134
 13515. Queued up writeback on the new page is triggered.
 136
 13716. 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
 14119. 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
 14520. The new page is moved to the LRU and can be scanned by the swapper
 146    etc again.
 147
 148Christoph Lameter, May 8, 2006.
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