This tunable allows someone to set more or less aggressive approaches to reclaim memory when a zone runs out of memory. If it is set to zero then no zone reclaim occurs. Allocations will be satisfied from other zones / nodes in the system.
This is value OR-ed together of
1 = Zone reclaim on
2 = Zone reclaim writes dirty pages out
4 = Zone reclaim swaps pages
zone_reclaim_mode is set during bootup to 1 if it is determined that pages from remote zones will cause a measurable performance reduction. The page allocator will then reclaim easily reusable pages (those page cache pages that are currently not used) before allocating off node pages.
It may be beneficial to switch off zone reclaim if the system is used for a file server and all of memory should be used for caching files from disk. In that case the caching effect is more important than data locality.
Allowing zone reclaim to write out pages stops processes that are writing large amounts of data from dirtying pages on other nodes. Zone reclaim will write out dirty pages if a zone fills up and so effectively throttle the process. This may decrease the performance of a single process since it cannot use all of system memory to buffer the outgoing writes anymore but it preserves the memory on other nodes so that the performance of other processes running on other nodes will not be affected.
Allowing regular swap effectively restricts allocations to the local node unless explicitly overridden by memory policies or cpuset configurations.