Re: [PATCH 36/46] mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task<->node relationships
From: Ingo Molnar <mingo@kernel.org>
Date: 2012-11-21 18:25:45
Also in:
lkml
Subsystem:
memory management, memory management - memory policy and migration, the rest · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds
* Mel Gorman [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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> --- mm/mempolicy.c | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-)diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 4c1c8d8..fd20e28 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c@@ -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:
As mentioned in my other mail, this patch of yours looks very similar to the numa/core commit attached below, mostly written by Peter: 30f93abc6cb3 sched, numa, mm: Add the scanning page fault machinery Thanks, Ingo --------------------->