On Wed, Nov 21, 2012 at 07:25:37PM +0100, Ingo Molnar wrote:
>
> * Mel Gorman <mgorman@xxxxxxx> wrote:
>
> > 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@xxxxxxx>
> > ---
> > 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
>
My patch is directly based on that particular patch and is a partial
extraction. I could not directly pull which is why the From is missing. I
think you'll also find that it's very similar to a partial extraction
from "autonuma: memory follows CPU algorithm and task/mm_autonuma stats
collection". The primary differences are exactly how the logic is applied
and when it happens.
I've added a note now to that effect now. For all the patches with notes
or any other ones, I'll be very happy to add the Signed-offs back on if
the original authors acknowledge they are ok with the end result. If you
recall, in the original V1 of this series I said;
This series steals very heavily from both autonuma and schednuma
with very little original code. In some cases I removed the
signed-off-bys because the result was too different. I have noted
in the changelog where this happened but the signed-offs can be
restored if the original authors agree.
Just to compare, this is the wording in "autonuma: memory follows CPU
algorithm and task/mm_autonuma stats collection"
+/*
+ * In this function we build a temporal CPU_node<->page relation by
+ * using a two-stage autonuma_last_nid filter to remove short/unlikely
+ * relations.
+ *
+ * Using P(p) ~ n_p / n_t as per frequentest probability, we can
+ * equate a node's CPU 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 then 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 CPU_node<->page relation.
+ */
If this was the basis for the sched/numa patch then I'd point out that
I'm not the only person that failed to preserve history perfectly.
--
Mel Gorman
SUSE Labs
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