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[2003:c4:4723:ccf5:e7a:c8e3:6571:71f5]) by smtp.gmail.com with ESMTPSA id h135sm24452892wmd.21.2019.02.06.06.31.18 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 06 Feb 2019 06:31:18 -0800 (PST) Date: Wed, 6 Feb 2019 15:31:17 +0100 From: Johannes Weiner To: Chris Down Cc: Andrew Morton , Michal Hocko , Tejun Heo , Roman Gushchin , Dennis Zhou , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com Subject: Re: [PATCH v4] mm: Make memory.emin the baseline for utilisation determination Message-ID: <20190206143117.GA30357@cmpxchg.org> References: <20190129191525.GB10430@chrisdown.name> <20190201051810.GA18895@chrisdown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190201051810.GA18895@chrisdown.name> User-Agent: Mutt/1.11.2 (2019-01-07) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 01, 2019 at 12:18:10AM -0500, Chris Down wrote: > Roman points out that when when we do the low reclaim pass, we scale the > reclaim pressure relative to position between 0 and the maximum > protection threshold. > > However, if the maximum protection is based on memory.elow, and > memory.emin is above zero, this means we still may get binary behaviour > on second-pass low reclaim. This is because we scale starting at 0, not > starting at memory.emin, and since we don't scan at all below emin, we > end up with cliff behaviour. > > This should be a fairly uncommon case since usually we don't go into the > second pass, but it makes sense to scale our low reclaim pressure > starting at emin. > > You can test this by catting two large sparse files, one in a cgroup > with emin set to some moderate size compared to physical RAM, and > another cgroup without any emin. In both cgroups, set an elow larger > than 50% of physical RAM. The one with emin will have less page > scanning, as reclaim pressure is lower. > > Signed-off-by: Chris Down > Suggested-by: Roman Gushchin > Cc: Johannes Weiner > Cc: Andrew Morton > Cc: Michal Hocko > Cc: Tejun Heo > Cc: Roman Gushchin > Cc: Dennis Zhou > Cc: linux-kernel@vger.kernel.org > Cc: cgroups@vger.kernel.org > Cc: linux-mm@kvack.org > Cc: kernel-team@fb.com Acked-by: Johannes Weiner