file: /proc/sys/vm/zone_reclaim_mode
variable: vm.zone_reclaim_mode
Official reference

Zone_reclaim_mode 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 ORed together of

  • 1 = Zone reclaim on
  • 2 = Zone reclaim writes dirty pages out
  • 4 = Zone reclaim swaps pages

zone_reclaim_mode is disabled by default. For file servers or workloads that benefit from having their data cached, zone_reclaim_mode should be left disabled as the caching effect is likely to be more important than data locality.

zone_reclaim may be enabled if it’s known that the workload is partitioned such that each partition fits within a NUMA node and that accessing remote memory would 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.

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 preserve 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.