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Page migration
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--------------
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Page migration allows the moving of the physical location of pages between
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nodes in a numa system while the process is running. This means that the
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virtual addresses that the process sees do not change. However, the
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system rearranges the physical location of those pages.
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The main intend of page migration is to reduce the latency of memory access
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by moving pages near to the processor where the process accessing that memory
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is running.
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Page migration allows a process to manually relocate the node on which its
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pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
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a new memory policy via mbind(). The pages of process can also be relocated
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from another process using the sys_migrate_pages() function call. The
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migrate_pages function call takes two sets of nodes and moves pages of a
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process that are located on the from nodes to the destination nodes.
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Page migration functions are provided by the numactl package by Andi Kleen
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(a version later than 0.9.3 is required. Get it from
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ftp://ftp.suse.com/pub/people/ak). numactl provided libnuma which
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provides an interface similar to other numa functionality for page migration.
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cat /proc/<pid>/numa_maps allows an easy review of where the pages of
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a process are located. See also the numa_maps manpage in the numactl package.
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Manual migration is useful if for example the scheduler has relocated
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a process to a processor on a distant node. A batch scheduler or an
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administrator may detect the situation and move the pages of the process
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nearer to the new processor. At some point in the future we may have
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some mechanism in the scheduler that will automatically move the pages.
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Larger installations usually partition the system using cpusets into
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sections of nodes. Paul Jackson has equipped cpusets with the ability to
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move pages when a task is moved to another cpuset (See ../cpusets.txt).
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Cpusets allows the automation of process locality. If a task is moved to
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a new cpuset then also all its pages are moved with it so that the
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performance of the process does not sink dramatically. Also the pages
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of processes in a cpuset are moved if the allowed memory nodes of a
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cpuset are changed.
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Page migration allows the preservation of the relative location of pages
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within a group of nodes for all migration techniques which will preserve a
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particular memory allocation pattern generated even after migrating a
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process. This is necessary in order to preserve the memory latencies.
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Processes will run with similar performance after migration.
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Page migration occurs in several steps. First a high level
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description for those trying to use migrate_pages() from the kernel
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(for userspace usage see the Andi Kleen's numactl package mentioned above)
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and then a low level description of how the low level details work.
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A. In kernel use of migrate_pages()
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-----------------------------------
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1. Remove pages from the LRU.
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Lists of pages to be migrated are generated by scanning over
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pages and moving them into lists. This is done by
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calling isolate_lru_page().
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Calling isolate_lru_page increases the references to the page
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so that it cannot vanish while the page migration occurs.
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It also prevents the swapper or other scans to encounter
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the page.
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2. Generate a list of newly allocates page. These pages will contain the
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contents of the pages from the first list after page migration is
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complete.
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3. The migrate_pages() function is called which attempts
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to do the migration. It returns the moved pages in the
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list specified as the third parameter and the failed
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migrations in the fourth parameter. The first parameter
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will contain the pages that could still be retried.
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4. The leftover pages of various types are returned
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to the LRU using putback_to_lru_pages() or otherwise
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disposed of. The pages will still have the refcount as
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increased by isolate_lru_pages() if putback_to_lru_pages() is not
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used! The kernel may want to handle the various cases of failures in
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different ways.
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B. How migrate_pages() works
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----------------------------
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migrate_pages() does several passes over its list of pages. A page is moved
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if all references to a page are removable at the time. The page has
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already been removed from the LRU via isolate_lru_page() and the refcount
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is increased so that the page cannot be freed while page migration occurs.
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Steps:
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1. Lock the page to be migrated
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2. Insure that writeback is complete.
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3. Make sure that the page has assigned swap cache entry if
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it is an anonyous page. The swap cache reference is necessary
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to preserve the information contain in the page table maps while
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page migration occurs.
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4. Prep the new page that we want to move to. It is locked
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and set to not being uptodate so that all accesses to the new
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page immediately lock while the move is in progress.
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5. All the page table references to the page are either dropped (file
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backed pages) or converted to swap references (anonymous pages).
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This should decrease the reference count.
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6. The radix tree lock is taken. This will cause all processes trying
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to reestablish a pte to block on the radix tree spinlock.
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7. The refcount of the page is examined and we back out if references remain
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otherwise we know that we are the only one referencing this page.
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8. The radix tree is checked and if it does not contain the pointer to this
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page then we back out because someone else modified the mapping first.
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9. The mapping is checked. If the mapping is gone then a truncate action may
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be in progress and we back out.
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10. The new page is prepped with some settings from the old page so that
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accesses to the new page will be discovered to have the correct settings.
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11. The radix tree is changed to point to the new page.
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12. The reference count of the old page is dropped because the radix tree
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reference is gone.
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13. The radix tree lock is dropped. With that lookups become possible again
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and other processes will move from spinning on the tree lock to sleeping on
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the locked new page.
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14. The page contents are copied to the new page.
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15. The remaining page flags are copied to the new page.
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16. The old page flags are cleared to indicate that the page does
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not use any information anymore.
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17. Queued up writeback on the new page is triggered.
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18. If swap pte's were generated for the page then replace them with real
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ptes. This will reenable access for processes not blocked by the page lock.
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19. The page locks are dropped from the old and new page.
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Processes waiting on the page lock can continue.
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20. The new page is moved to the LRU and can be scanned by the swapper
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etc again.
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TODO list
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---------
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- Page migration requires the use of swap handles to preserve the
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information of the anonymous page table entries. This means that swap
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space is reserved but never used. The maximum number of swap handles used
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is determined by CHUNK_SIZE (see mm/mempolicy.c) per ongoing migration.
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Reservation of pages could be avoided by having a special type of swap
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handle that does not require swap space and that would only track the page
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references. Something like that was proposed by Marcelo Tosatti in the
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past (search for migration cache on lkml or linux-mm@kvack.org).
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- Page migration unmaps ptes for file backed pages and requires page
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faults to reestablish these ptes. This could be optimized by somehow
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recording the references before migration and then reestablish them later.
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However, there are several locking challenges that have to be overcome
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before this is possible.
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- Page migration generates read ptes for anonymous pages. Dirty page
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faults are required to make the pages writable again. It may be possible
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to generate a pte marked dirty if it is known that the page is dirty and
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that this process has the only reference to that page.
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2006-03-15 04:50:20 +01:00
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Christoph Lameter, March 8, 2006.
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