linux-kernel - Re: [RFC PATCH 01/31] mm: migrate: Add exchange

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Message-ID: <20190219125619.GA12668@bombadil.infradead.org>
Date:   Tue, 19 Feb 2019 04:56:19 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Zi Yan <ziy@...dia.com>, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        John Hubbard <jhubbard@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Nitin Gupta <nigupta@...dia.com>,
        David Nellans <dnellans@...dia.com>
Subject: Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange
 two lists of pages.

On Tue, Feb 19, 2019 at 01:12:07PM +0530, Anshuman Khandual wrote:
> But the location of this temp page matters as well because you would like to
> saturate the inter node interface. It needs to be either of the nodes where
> the source or destination page belongs. Any other node would generate two
> internode copy process which is not what you intend here I guess.

That makes no sense.  It should be allocated on the local node of the CPU
performing the copy.  If the CPU is in node A, the destination is in node B
and the source is in node C, then you're doing 4k worth of reads from node C,
4k worth of reads from node B, 4k worth of writes to node C followed by
4k worth of writes to node B.  Eventually the 4k of dirty cachelines on
node A will be written back from cache to the local memory (... or not,
if that page gets reused for some other purpose first).

If you allocate the page on node B or node C, that's an extra 4k of writes
to be sent across the inter-node link.