Page migration allows a process to manually relocate the node on which its
pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
-a new memory policy. The pages of process can also be relocated
+a new memory policy via mbind(). The pages of process can also be relocated
from another process using the sys_migrate_pages() function call. The
migrate_pages function call takes two sets of nodes and moves pages of a
process that are located on the from nodes to the destination nodes.
-
-Manual migration is very useful if for example the scheduler has relocated
+Page migration functions are provided by the numactl package by Andi Kleen
+(a version later than 0.9.3 is required. Get it from
+ftp://ftp.suse.com/pub/people/ak). numactl provided libnuma which
+provides an interface similar to other numa functionality for page migration.
+cat /proc/<pid>/numa_maps allows an easy review of where the pages of
+a process are located. See also the numa_maps manpage in the numactl package.
+
+Manual migration is useful if for example the scheduler has relocated
a process to a processor on a distant node. A batch scheduler or an
administrator may detect the situation and move the pages of the process
-nearer to the new processor. At some point in the future we may have
-some mechanism in the scheduler that will automatically move the pages.
+nearer to the new processor. The kernel itself does only provide
+manual page migration support. Automatic page migration may be implemented
+through user space processes that move pages. A special function call
+"move_pages" allows the moving of individual pages within a process.
+A NUMA profiler may f.e. obtain a log showing frequent off node
+accesses and may use the result to move pages to more advantageous
+locations.
Larger installations usually partition the system using cpusets into
sections of nodes. Paul Jackson has equipped cpusets with the ability to
-move pages when a task is moved to another cpuset. This allows automatic
-control over locality of a process. If a task is moved to a new cpuset
-then also all its pages are moved with it so that the performance of the
-process does not sink dramatically (as is the case today).
+move pages when a task is moved to another cpuset (See ../cpusets.txt).
+Cpusets allows the automation of process locality. If a task is moved to
+a new cpuset then also all its pages are moved with it so that the
+performance of the process does not sink dramatically. Also the pages
+of processes in a cpuset are moved if the allowed memory nodes of a
+cpuset are changed.
Page migration allows the preservation of the relative location of pages
within a group of nodes for all migration techniques which will preserve a
Processes will run with similar performance after migration.
Page migration occurs in several steps. First a high level
-description for those trying to use migrate_pages() and then
-a low level description of how the low level details work.
+description for those trying to use migrate_pages() from the kernel
+(for userspace usage see the Andi Kleen's numactl package mentioned above)
+and then a low level description of how the low level details work.
-A. Use of migrate_pages()
--------------------------
+A. In kernel use of migrate_pages()
+-----------------------------------
1. Remove pages from the LRU.
Lists of pages to be migrated are generated by scanning over
pages and moving them into lists. This is done by
- calling isolate_lru_page() or __isolate_lru_page().
+ calling isolate_lru_page().
Calling isolate_lru_page increases the references to the page
- so that it cannot vanish under us.
+ so that it cannot vanish while the page migration occurs.
+ It also prevents the swapper or other scans to encounter
+ the page.
-2. Generate a list of newly allocates page to move the contents
- of the first list to.
+2. We need to have a function of type new_page_t that can be
+ passed to migrate_pages(). This function should figure out
+ how to allocate the correct new page given the old page.
3. The migrate_pages() function is called which attempts
- to do the migration. It returns the moved pages in the
- list specified as the third parameter and the failed
- migrations in the fourth parameter. The first parameter
- will contain the pages that could still be retried.
-
-4. The leftover pages of various types are returned
- to the LRU using putback_to_lru_pages() or otherwise
- disposed of. The pages will still have the refcount as
- increased by isolate_lru_pages()!
+ to do the migration. It will call the function to allocate
+ the new page for each page that is considered for
+ moving.
-B. Operation of migrate_pages()
---------------------------------
+B. How migrate_pages() works
+----------------------------
-migrate_pages does several passes over its list of pages. A page is moved
-if all references to a page are removable at the time.
+migrate_pages() does several passes over its list of pages. A page is moved
+if all references to a page are removable at the time. The page has
+already been removed from the LRU via isolate_lru_page() and the refcount
+is increased so that the page cannot be freed while page migration occurs.
Steps:
2. Insure that writeback is complete.
-3. Make sure that the page has assigned swap cache entry if
- it is an anonyous page. The swap cache reference is necessary
- to preserve the information contain in the page table maps.
-
-4. Prep the new page that we want to move to. It is locked
+3. Prep the new page that we want to move to. It is locked
and set to not being uptodate so that all accesses to the new
- page immediately lock while we are moving references.
+ page immediately lock while the move is in progress.
-5. All the page table references to the page are either dropped (file backed)
- or converted to swap references (anonymous pages). This should decrease the
- reference count.
+4. The new page is prepped with some settings from the old page so that
+ accesses to the new page will discover a page with the correct settings.
-6. The radix tree lock is taken
+5. All the page table references to the page are converted
+ to migration entries or dropped (nonlinear vmas).
+ This decrease the mapcount of a page. If the resulting
+ mapcount is not zero then we do not migrate the page.
+ All user space processes that attempt to access the page
+ will now wait on the page lock.
+
+6. The radix tree lock is taken. This will cause all processes trying
+ to access the page via the mapping to block on the radix tree spinlock.
7. The refcount of the page is examined and we back out if references remain
otherwise we know that we are the only one referencing this page.
8. The radix tree is checked and if it does not contain the pointer to this
- page then we back out.
-
-9. The mapping is checked. If the mapping is gone then a truncate action may
- be in progress and we back out.
-
-10. The new page is prepped with some settings from the old page so that accesses
- to the new page will be discovered to have the correct settings.
+ page then we back out because someone else modified the radix tree.
-11. The radix tree is changed to point to the new page.
+9. The radix tree is changed to point to the new page.
-12. The reference count of the old page is dropped because the reference has now
- been removed.
+10. The reference count of the old page is dropped because the radix tree
+ reference is gone. A reference to the new page is established because
+ the new page is referenced to by the radix tree.
-13. The radix tree lock is dropped.
+11. The radix tree lock is dropped. With that lookups in the mapping
+ become possible again. Processes will move from spinning on the tree_lock
+ to sleeping on the locked new page.
-14. The page contents are copied to the new page.
+12. The page contents are copied to the new page.
-15. The remaining page flags are copied to the new page.
+13. The remaining page flags are copied to the new page.
-16. The old page flags are cleared to indicate that the page does
- not use any information anymore.
+14. The old page flags are cleared to indicate that the page does
+ not provide any information anymore.
-17. Queued up writeback on the new page is triggered.
+15. Queued up writeback on the new page is triggered.
-18. If swap pte's were generated for the page then remove them again.
+16. If migration entries were page then replace them with real ptes. Doing
+ so will enable access for user space processes not already waiting for
+ the page lock.
-19. The locks are dropped from the old and new page.
+19. The page locks are dropped from the old and new page.
+ Processes waiting on the page lock will redo their page faults
+ and will reach the new page.
-20. The new page is moved to the LRU.
+20. The new page is moved to the LRU and can be scanned by the swapper
+ etc again.
-Christoph Lameter, December 19, 2005.
+Christoph Lameter, May 8, 2006.
projects / linux-drm-fsl-dcu.git / blobdiff