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-Page migration
---------------
-
-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
-is running.
-
-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
-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 (See
-Documentation/cgroups/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
-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
- moving.
-
-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.
-
-Steps:
-
-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.
-