Re: [PATCH 27/50] mm: numa: Scan pages with elevated page_mapcount

From: Hillf Danton
Date: Wed Sep 11 2013 - 22:10:36 EST


Hillo Mel

On Tue, Sep 10, 2013 at 5:32 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> Currently automatic NUMA balancing is unable to distinguish between false
> shared versus private pages except by ignoring pages with an elevated
> page_mapcount entirely. This avoids shared pages bouncing between the
> nodes whose task is using them but that is ignored quite a lot of data.
>
> This patch kicks away the training wheels in preparation for adding support
> for identifying shared/private pages is now in place. The ordering is so
> that the impact of the shared/private detection can be easily measured. Note
> that the patch does not migrate shared, file-backed within vmas marked
> VM_EXEC as these are generally shared library pages. Migrating such pages
> is not beneficial as there is an expectation they are read-shared between
> caches and iTLB and iCache pressure is generally low.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
[...]
> @@ -1658,13 +1660,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> int page_lru = page_is_file_cache(page);
>
> /*
> - * Don't migrate pages that are mapped in multiple processes.
> - * TODO: Handle false sharing detection instead of this hammer
> - */
> - if (page_mapcount(page) != 1)
> - goto out_dropref;
> -
Is there rmap walk when migrating THP?

> - /*
> * Rate-limit the amount of data that is being migrated to a node.
> * Optimal placement is no good if the memory bus is saturated and
> * all the time is being spent migrating!
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