linux-kernel - Re: [PATCH v31 02/13] mm/damon/core: Implement region-based sampling

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Date:   Tue, 22 Jun 2021 07:59:19 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     SeongJae Park <sj38.park@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sjpark@...zon.de>, Jonathan.Cameron@...wei.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        amit@...nel.org, benh@...nel.crashing.org,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        David Hildenbrand <david@...hat.com>, dwmw@...zon.com,
        Marco Elver <elver@...gle.com>, "Du, Fan" <fan.du@...el.com>,
        foersleo@...zon.de, greg@...ah.com,
        Greg Thelen <gthelen@...gle.com>, guoju.fgj@...baba-inc.com,
        jgowans@...zon.com, Mel Gorman <mgorman@...e.de>, mheyne@...zon.de,
        Minchan Kim <minchan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, namhyung@...nel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        David Rientjes <rientjes@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mike Rapoport <rppt@...nel.org>, Shuah Khan <shuah@...nel.org>,
        sieberf@...zon.com, snu@...le79.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        zgf574564920@...il.com, linux-damon@...zon.com,
        Linux MM <linux-mm@...ck.org>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v31 02/13] mm/damon/core: Implement region-based sampling

On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@...il.com> wrote:
>
> From: SeongJae Park <sjpark@...zon.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@...zon.de>
> Reviewed-by: Leonard Foerster <foersleo@...zon.de>
> Reviewed-by: Fernand Sieber <sieberf@...zon.com>

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