KernelTrap: [Linux] Compacting Memory

Mel Gorman offered a first release of a patchset that compacts memory, "this is a prototype for compacting memory to reduce external fragmentation so that free memory exists as fewer, but larger contiguous blocks. Rather than being a full defragmentation solution, this focuses exclusively on pages that are movable via the page migration mechanism." He notes that the patchset is currently incomplete, and at this time memory is only compacted manually, not automatically, "this version of the patchset is mainly concerned with getting the compaction mechanism correct." Mel goes on to describe how it works:

"A single compaction run involves two scanners operating within a zone - a migration and a free scanner. The migration scanner starts at the beginning of a zone and finds all movable pages within one pageblock_nr_pages-sized area and isolates them on a migratepages list. The free scanner begins at the end of the zone and searches on a per-area basis for enough free pages to migrate all the pages on the migratepages list. As each area is respectively migrated or exhausted of free pages, the scanners are advanced one area. A compaction run completes within a zone when the two scanners meet."

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