KernelTrap - RAM

Linux: Memory Compaction

Jeremy — Sat, 03 Apr 2010 00:11:38 +0000

Mel Gorman posted the seventh version of his Memory Compaction patches asking, "are there any further obstacles to merging?" The patches, first posted in May of 2007, provide a mechanism for moving GFP_MOVABLE pages into a smaller number of pageblocks, reducing externally fragmented memory. Mel explains that 'compaction' is another method of defragmenting memory, "for example, lumpy reclaim is a form of defragmentation as was slub 'defragmentation' (really a form of targeted reclaim). Hence, this is called 'compaction' to distinguish it from other forms of defragmentation."

The core compaction patch explains that memory is compacted in a zone by relocating movable pages towards the end of the zone:

"A single compaction run involves a migration scanner and a free scanner. Both scanners operate on pageblock-sized areas in the zone. The migration scanner starts at the bottom of the zone and searches for all movable pages within each area, isolating them onto a private list called migratelist. The free scanner starts at the top of the zone and searches for suitable areas and consumes the free pages within making them available for the migration scanner. The pages isolated for migration are then migrated to the newly isolated free pages."

Keeping Ramdisk Pages Dirty

Jeremy — Thu, 11 Oct 2007 20:01:06 +0000

"We have seen ramdisk based install systems, where some pages of mapped libraries and programs were suddendly zeroed under memory pressure. This should not happen, as the ramdisk avoids freeing its pages by keeping them dirty all the time," Christian Borntraeger began, explaining the need for his small patch to the ramdisk driver. He continued, "it turns out that there is a case, where the VM makes a ramdisk page clean, without telling the ramdisk driver. On memory pressure shrink_zone runs and it starts to run shrink_active_list. There is a check for buffer_heads_over_limit, and if true, pagevec_strip is called. pagevec_strip calls try_to_release_page. If the mapping has no releasepage callback, try_to_free_buffers is called. try_to_free_buffers has now a special logic for some file systems to make a dirty page clean, if all buffers are clean. Thats what happened in our test case."

He provided two methods for duplicating the reported problem, "you have to make buffer_heads_over_limit true" This is done by either lowering max_buffer_heads or having a system with lots of high memory. "The solution is to provide a noop-releasepage callback for the ramdisk driver. This avoids try_to_free_buffers for ramdisk pages."