linux-kernel - Re: [PATCH v13 04/15] mm/damon: Implement region based sampling

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Date:   Wed, 27 May 2020 13:23:48 +0200
From:   Leonard Foerster <foersleo@...zon.com>
To:     SeongJae Park <sjpark@...zon.com>
Cc:     akpm@...ux-foundation.org, SeongJae Park <sjpark@...zon.de>,
        Jonathan.Cameron@...wei.com, aarcange@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, amit@...nel.org,
        benh@...nel.crashing.org, brendan.d.gregg@...il.com,
        brendanhiggins@...gle.com, cai@....pw, colin.king@...onical.com,
        corbet@....net, dwmw@...zon.com, irogers@...gle.com,
        jolsa@...hat.com, kirill@...temov.name, mark.rutland@....com,
        mgorman@...e.de, minchan@...nel.org, mingo@...hat.com,
        namhyung@...nel.org, peterz@...radead.org, rdunlap@...radead.org,
        riel@...riel.com, rientjes@...gle.com, rostedt@...dmis.org,
        sblbir@...zon.com, shakeelb@...gle.com, shuah@...nel.org,
        sj38.park@...il.com, snu@...zon.de, vbabka@...e.cz,
        vdavydov.dev@...il.com, yang.shi@...ux.alibaba.com,
        ying.huang@...el.com, linux-damon@...zon.com, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 04/15] mm/damon: Implement region based sampling

On 2020-05-25T11:15:01+02:00 SeongJae Park <sjpark@...zon.com> wrote:

> From: SeongJae Park <sjpark@...zon.de>
> 
> This commit implements DAMON's basic access check and region based
> sampling mechanisms.  This change would seems make no sense, mainly
> because it is only a part of the DAMON's logics.  Following two commits
> will make more sense.
> 
> Basic Access Check
> ------------------
> 
> DAMON basically reports what pages are how frequently accessed.  Note
> that the frequency is not an absolute number of accesses, but a relative
> frequency among the pages of the target workloads.
> 
> Users can control the resolution of the reports by setting two time
> intervals, ``sampling interval`` and ``aggregation interval``.  In
> detail, DAMON checks access to each page per ``sampling interval``,
> aggregates the results (counts the number of the accesses to each page),
> and reports the aggregated results per ``aggregation interval``.  For
> the access check of each page, DAMON uses the Accessed bits of PTEs.
> 
> This is thus similar to common periodic access checks based access
> tracking mechanisms, which overhead is increasing as the size of the
> target process grows.
> 
> Region Based Sampling
> ---------------------
> 
> To avoid the unbounded increase of the overhead, DAMON groups a number
> of adjacent pages that assumed to have same access frequencies into a
> region.  As long as the assumption (pages in a region have same access
> frequencies) is kept, only one page in the region is required to be
> checked.  Thus, for each ``sampling interval``, DAMON randomly picks one
> page in each region and clears its Accessed bit.  After one more
> ``sampling interval``, DAMON reads the Accessed bit of the page and
> increases the access frequency of the region if the bit has set
> meanwhile.  Therefore, the monitoring overhead is controllable by
> setting the number of regions.
> 
> Nonetheless, this scheme cannot preserve the quality of the output if
> the assumption is not kept.  Following commit will introduce how we can
> make the guarantee with best effort.
> 
> Signed-off-by: SeongJae Park <sjpark@...zon.de>

Reviewed-by: Leonard Foerster <foersleo@...zon.de>