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130 lines
5.1 KiB
Plaintext
130 lines
5.1 KiB
Plaintext
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. 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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Manual migration is very 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. This allows automatic
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control over locality of a process. If a task is moved to a new cpuset
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then also all its pages are moved with it so that the performance of the
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process does not sink dramatically (as is the case today).
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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() and then
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a low level description of how the low level details work.
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A. 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() or __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 under us.
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2. Generate a list of newly allocates page to move the contents
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of the first list to.
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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()!
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B. Operation of migrate_pages()
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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.
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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.
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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 we are moving references.
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5. All the page table references to the page are either dropped (file backed)
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or converted to swap references (anonymous pages). This should decrease the
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reference count.
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6. The radix tree lock is taken
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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.
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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 accesses
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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 reference has now
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been removed.
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13. The radix tree lock is dropped.
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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 remove them again.
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19. The locks are dropped from the old and new page.
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20. The new page is moved to the LRU.
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Christoph Lameter, December 19, 2005.
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