linux-kernel - Re: [PATCH 0/1] Add swappiness argument to memory.reclaim

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Date:   Thu, 30 Nov 2023 16:57:41 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Dan Schatzberg <schatzberg.dan@...il.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Yosry Ahmed <yosryahmed@...gle.com>, Huan Yang <link@...o.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Huang Ying <ying.huang@...el.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Peter Xu <peterx@...hat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 0/1] Add swappiness argument to memory.reclaim

On Thu 30-11-23 07:36:53, Dan Schatzberg wrote:
[...]
> In contrast, I argue in favor of a swappiness setting not as a way to implement
> custom reclaim algorithms but rather to bias the balance of anon vs file due to
> differences of proactive vs reactive reclaim. In this context, swappiness is the
> existing interface for controlling this balance and this patch simply allows for
> it to be configured differently for proactive vs reactive reclaim.

I do agree that swappiness is a better interface than explicit anon/file
but the problem with swappiness is that it is more of a hint for the reclaim
rather than a real control. Just look at get_scan_count and its history.
Not only its range has been extended also the extent when it is actually
used has been changing all the time and I think it is not a stretch to
assume that trend to continue.

Now if we extend the user interface to trigger the reclaim do we expect
that we always do SCAN_EQUAL if a user specifies swappiness or are we OK
that the implementation is free to ignore that "hint"?
-- 
Michal Hocko
SUSE Labs