[PATCH] Zone reclaim: Allow modification of zone reclaim behavior

zone_reclaim_mode:

This is set during bootup to 1 if it is determined that pages from

remote zones will cause a significant performance reduction. The

Zone_reclaim_mode allows to set more or less agressive 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 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 going off node.

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 preserve the memory on other nodes so that the performance

of other processes running on other nodes will not be affected.

The user can override this setting. 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.

Allowing regular swap effectively restricts allocations to the local

node unless explicitly overridden by memory policies or cpuset

configurations.

It may be beneficial to switch this on if one wants to do zone

reclaim regardless of the numa distances in the system.

================================================================

zone_reclaim_interval:

 */

int zone_reclaim_mode __read_mostly;

#define RECLAIM_OFF 0

#define RECLAIM_ZONE (1<<0)	/* Run shrink_cache on the zone */

#define RECLAIM_WRITE (1<<1)	/* Writeout pages during reclaim */

#define RECLAIM_SWAP (1<<2)	/* Swap pages out during reclaim */

/*

 * Mininum time between zone reclaim scans

 */

	if (!cpus_empty(mask) && node_id != numa_node_id())

		return 0;

	sc.may_writepage = 0;

	sc.may_swap = 0;

	sc.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE);

	sc.may_swap = !!(zone_reclaim_mode & RECLAIM_SWAP);

	sc.nr_scanned = 0;

	sc.nr_reclaimed = 0;

	sc.priority = ZONE_RECLAIM_PRIORITY + 1;