|Page migration allows the moving of the physical location of pages between
|nodes in a numa system while the process is running. This means that the
|virtual addresses that the process sees do not change. However, the
|system rearranges the physical location of those pages.
|The main intend of page migration is to reduce the latency of memory access
|by moving pages near to the processor where the process accessing that memory
|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 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.
|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://oss.sgi.com/www/projects/libnuma/download/). numactl provides 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 documentation in the
|proc(5) man page.
|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. 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
|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 (See
|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
|particular memory allocation pattern generated even after migrating a
|process. This is necessary in order to preserve the memory latencies.
|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() 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. 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().
| Calling isolate_lru_page increases the references to the page
| so that it cannot vanish while the page migration occurs.
| It also prevents the swapper or other scans to encounter
| the page.
|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 will call the function to allocate
| the new page for each page that is considered for
|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. 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.
|1. Lock the page to be migrated
|2. Insure that writeback is complete.
|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 the move is in progress.
|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.
|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 because someone else modified the radix tree.
|9. The radix tree is changed to point to the new page.
|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.
|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.
|12. The page contents are copied to the new page.
|13. The remaining page flags are copied to the new page.
|14. The old page flags are cleared to indicate that the page does
| not provide any information anymore.
|15. Queued up writeback on the new page is triggered.
|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 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 and can be scanned by the swapper
| etc again.
|Christoph Lameter, May 8, 2006.