Documentation/vm/page_migration

   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
  15 a new memory policy via mbind(). The pages of process can also be relocated
  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.
  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
  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
  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.
  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
  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.
  51
  52 A. In kernel use of migrate_pages()
  53 -----------------------------------
  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
  59    calling isolate_lru_page().
  60    Calling isolate_lru_page increases the references to the page
  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.
  64
  65 2. Generate a list of newly allocates pages. These pages will contain the
  66    contents of the pages from the first list after page migration is
  67    complete.
  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. When the function
  73    returns the first list 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
  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.
  81
  82 B. How migrate_pages() works
  83 ----------------------------
  84
  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.
  89
  90 Steps:
  91
  92 1. Lock the page to be migrated
  93
  94 2. Insure that writeback is complete.
  95
  96 3. Prep the new page that we want to move to. It is locked
  97    and set to not being uptodate so that all accesses to the new
  98    page immediately lock while the move is in progress.
  99
 100 4. The new page is prepped with some settings from the old page so that
 101    accesses to the new page will discover a page with the correct settings.
 102
 103 5. All the page table references to the page are converted
 104    to migration entries or dropped (nonlinear vmas).
 105    This decrease the mapcount of a page. If the resulting
 106    mapcount is not zero then we do not migrate the page.
 107    All user space processes that attempt to access the page
 108    will now wait on the page lock.
 109
 110 6. The radix tree lock is taken. This will cause all processes trying
 111    to access the page via the mapping to block on the radix tree spinlock.
 112
 113 7. The refcount of the page is examined and we back out if references remain
 114    otherwise we know that we are the only one referencing this page.
 115
 116 8. The radix tree is checked and if it does not contain the pointer to this
 117    page then we back out because someone else modified the radix tree.
 118
 119 9. The radix tree is changed to point to the new page.
 120
 121 10. The reference count of the old page is dropped because the radix tree
 122     reference is gone. A reference to the new page is established because
 123     the new page is referenced to by the radix tree.
 124
 125 11. The radix tree lock is dropped. With that lookups in the mapping
 126     become possible again. Processes will move from spinning on the tree_lock
 127     to sleeping on the locked new page.
 128
 129 12. The page contents are copied to the new page.
 130
 131 13. The remaining page flags are copied to the new page.
 132
 133 14. The old page flags are cleared to indicate that the page does
 134     not provide any information anymore.
 135
 136 15. Queued up writeback on the new page is triggered.
 137
 138 16. If migration entries were page then replace them with real ptes. Doing
 139     so will enable access for user space processes not already waiting for
 140     the page lock.
 141
 142 19. The page locks are dropped from the old and new page.
 143     Processes waiting on the page lock will redo their page faults
 144     and will reach the new page.
 145
 146 20. The new page is moved to the LRU and can be scanned by the swapper
 147     etc again.
 148
 149 Christoph Lameter, May 8, 2006.
 150