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Fri, 01 Dec 2023 07:49:54 -0800 (PST) Received: from dschatzberg-fedora-PC0Y6AEN ([2620:10d:c091:400::5:29e2]) by smtp.gmail.com with ESMTPSA id b14-20020a0cc98e000000b0067a1a5abae0sm1578209qvk.93.2023.12.01.07.49.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 Dec 2023 07:49:53 -0800 (PST) Date: Fri, 1 Dec 2023 10:49:51 -0500 From: Dan Schatzberg To: Michal Hocko Cc: Johannes Weiner , Roman Gushchin , Yosry Ahmed , Huan Yang , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Shakeel Butt , Muchun Song , Andrew Morton , David Hildenbrand , Matthew Wilcox , Huang Ying , Kefeng Wang , Peter Xu , "Vishal Moola (Oracle)" , Yue Zhao , Hugh Dickins Subject: Re: [PATCH 0/1] Add swappiness argument to memory.reclaim Message-ID: References: <20231130153658.527556-1-schatzberg.dan@gmail.com> <20231130165642.GA386439@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: BF365180027 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: fofytewgsdkkw7iowjqu6ask1mmcwcm9 X-HE-Tag: 1701445795-869152 X-HE-Meta: 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 aNZJ7lYh 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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.