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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 22 Jun 2021 07:59:19 -0700
Message-ID: <CALvZod7mJivKs+u8cTKrYuUg=X+XFjXZ3zBnureFzTqK+BLrYQ@mail.gmail.com>
Subject: Re: [PATCH v31 02/13] mm/damon/core: Implement region-based sampling
To:     SeongJae Park <sj38.park@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        SeongJae Park <sjpark@amazon.de>, Jonathan.Cameron@huawei.com,
        acme@kernel.org, alexander.shishkin@linux.intel.com,
        amit@kernel.org, benh@kernel.crashing.org,
        Brendan Higgins <brendanhiggins@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        David Hildenbrand <david@redhat.com>, dwmw@amazon.com,
        Marco Elver <elver@google.com>, "Du, Fan" <fan.du@intel.com>,
        foersleo@amazon.de, greg@kroah.com,
        Greg Thelen <gthelen@google.com>, guoju.fgj@alibaba-inc.com,
        jgowans@amazon.com, Mel Gorman <mgorman@suse.de>, mheyne@amazon.de,
        Minchan Kim <minchan@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, namhyung@kernel.org,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
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        sieberf@amazon.com, snu@zelle79.org,
        Vlastimil Babka <vbabka@suse.cz>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        zgf574564920@gmail.com, linux-damon@amazon.com,
        Linux MM <linux-mm@kvack.org>, linux-doc@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@gmail.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.de>
>
> To avoid the unbounded increase of the overhead, DAMON groups adjacent
> pages that are assumed to have the same access frequencies into a
> region.  As long as the assumption (pages in a region have the same
> access frequencies) is kept, only one page in the region is required to
> be checked.  Thus, for each ``sampling interval``,
>
>  1. the 'prepare_access_checks' primitive picks one page in each region,
>  2. waits for one ``sampling interval``,
>  3. checks whether the page is accessed meanwhile, and
>  4. increases the access count of the region if so.
>
> Therefore, the monitoring overhead is controllable by adjusting the
> number of regions.  DAMON allows both the underlying primitives and user
> callbacks to adjust regions for the trade-off.  In other words, this
> commit makes DAMON to use not only time-based sampling but also
> space-based sampling.
>
> This scheme, however, cannot preserve the quality of the output if the
> assumption is not guaranteed.  Next commit will address this problem.
>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> Reviewed-by: Leonard Foerster <foersleo@amazon.de>
> Reviewed-by: Fernand Sieber <sieberf@amazon.com>

Acked-by: Shakeel Butt <shakeelb@google.com>