linux-kernel - Re: [PATCH 27/50] mm: numa: Scan pages with elevated page

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Date:	Thu, 12 Sep 2013 10:10:13 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 27/50] mm: numa: Scan pages with elevated page_mapcount

Hillo Mel

On Tue, Sep 10, 2013 at 5:32 PM, Mel Gorman <mgorman@...e.de> 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@...e.de>
> ---
[...]
> @@ -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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