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diff --git a/Documentation/vm/page_migration b/Documentation/vm/page_migration deleted file mode 100644 index 6513fe2d90b..00000000000 --- a/Documentation/vm/page_migration +++ /dev/null @@ -1,149 +0,0 @@ -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. - |