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mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task<->node relationships

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Note: This two-stage filter was taken directly from the sched/numa patch
	"sched, numa, mm: Add the scanning page fault machinery" but is
	only a partial extraction. As the end result is not necessarily
	recognisable, the signed-offs-by had to be removed. Will be added
	back if requested.

While it is desirable that all threads in a process run on its home
node, this is not always possible or necessary. There may be more
threads than exist within the node or the node might over-subscribed
with unrelated processes.

This can cause a situation whereby a page gets migrated off its home
node because the threads clearing pte_numa were running off-node. This
patch uses page->last_nid to build a two-stage filter before pages get
migrated to avoid problems with short or unlikely task<->node
relationships.

Signed-off-by: Mel Gorman <mgorman@suse.de>
This commit is contained in:
Mel Gorman 2012-11-12 09:17:07 +00:00
parent bac0382c6a
commit e42c8ff299
1 changed files with 29 additions and 1 deletions
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30
mm/mempolicy.c
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@ -2317,9 +2317,37 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
}
/* Migrate the page towards the node whose CPU is referencing it */
if (pol->flags & MPOL_F_MORON)
if (pol->flags & MPOL_F_MORON) {
int last_nid;
polnid = numa_node_id();
/*
* Multi-stage node selection is used in conjunction
* with a periodic migration fault to build a temporal
* task<->page relation. By using a two-stage filter we
* remove short/unlikely relations.
*
* Using P(p) ~ n_p / n_t as per frequentist
* probability, we can equate a task's usage of a
* particular page (n_p) per total usage of this
* page (n_t) (in a given time-span) to a probability.
*
* Our periodic faults will sample this probability and
* getting the same result twice in a row, given these
* samples are fully independent, is then given by
* P(n)^2, provided our sample period is sufficiently
* short compared to the usage pattern.
*
* This quadric squishes small probabilities, making
* it less likely we act on an unlikely task<->page
* relation.
*/
last_nid = page_xchg_last_nid(page, polnid);
if (last_nid != polnid)
goto out;
}
if (curnid != polnid)
ret = polnid;
out: