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Date:   Fri, 1 Dec 2023 10:49:51 -0500
From:   Dan Schatzberg <schatzberg.dan@gmail.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Yosry Ahmed <yosryahmed@google.com>, Huan Yang <link@vivo.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Huang Ying <ying.huang@intel.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Peter Xu <peterx@redhat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
        Yue Zhao <findns94@gmail.com>, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH 0/1] Add swappiness argument to memory.reclaim
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On Fri, Dec 01, 2023 at 10:33:01AM +0100, Michal Hocko wrote:
> On Thu 30-11-23 11:56:42, Johannes Weiner wrote:
> [...]
> > So I wouldn't say it's merely a reclaim hint. It controls a very
> > concrete and influential factor in VM decision making. And since the
> > global swappiness is long-established ABI, I don't expect its meaning
> > to change significantly any time soon.
> 
> As I've said I am more worried about potential future changes which
> would modify existing, reduce or add more corner cases which would be
> seen as a change of behavior from the user space POV. That means that we
> would have to be really explicit about the fact that the reclaim is free
> to override the swappiness provided by user. So essentially a best
> effort interface without any actual guarantees. That surely makes it
> harder to use. Is it still useable?

For our needs (limiting swapout and avoiding swap-depletion) we rely
on two semantics of vm.swappiness.

1) Lower swappiness results in less swap-out, more swappiness results
in more swap-out - for the same workload. Our proactive reclaimer
monitors swap-out and lowers swappiness in response if we exceed our
target swap-out rate.

2) swappiness = 0 results in no or very little swap-out. We rely on
this to avoid exhausting swap due to proactive reclaim and triggering
OOMs.

We already depend on these semantics of vm.swappiness *today*. I think
changing either of these would be seen as a behavior change from user
space POV irrespective of this patch. The proposal in this patch only
allows for vm.swappiness (whatever its semantics) to be configured
separately for proactive reclaim.