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[37.188.253.35]) by smtp.gmail.com with ESMTPSA id v10sm3832526wml.44.2020.03.12.02.25.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Mar 2020 02:25:32 -0700 (PDT) Date: Thu, 12 Mar 2020 10:25:31 +0100 From: Michal Hocko To: Ivan Teterevkov Cc: "corbet@lwn.net" , "akpm@linux-foundation.org" , "mchehab+samsung@kernel.org" , "tglx@linutronix.de" , "jpoimboe@redhat.com" , "pawan.kumar.gupta@linux.intel.com" , "jgross@suse.com" , "oneukum@suse.com" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: Re: [PATCH] mm/vmscan: add vm_swappiness configuration knobs Message-ID: <20200312092531.GU23944@dhcp22.suse.cz> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Wed 11-03-20 17:45:58, Ivan Teterevkov wrote: > This patch adds a couple of knobs: > > - The configuration option (CONFIG_VM_SWAPPINESS). > - The command line parameter (vm_swappiness). > > The default value is preserved, but now defined by CONFIG_VM_SWAPPINESS. > > Historically, the default swappiness is set to the well-known value 60, > and this works well for the majority of cases. The vm_swappiness is also > exposed as the kernel parameter that can be changed at runtime too, e.g. > with sysctl. > > This approach might not suit well some configurations, e.g. systemd-based > distros, where systemd is put in charge of the cgroup controllers, > including the memory one. In such cases, the default swappiness 60 > is copied across the cgroup subtrees early at startup, when systemd > is arranging the slices for its services, before the sysctl.conf > or tmpfiles.d/*.conf changes are applied. > > One could run a script to traverse the cgroup trees later and set the > desired memory.swappiness individually in each occurrence when the runtime > is set up, but this would require some amount of work to implement > properly. Instead, why not set the default swappiness as early as possible? I have to say I am not a great fan of more tunning for swappiness as this is quite a poor tunning for many years already. It essentially does nothing in many cases because the reclaim process ignores to value in many cases (have a look a get_scan_count. I have seen quite some reports that setting a specific value for vmswappiness didn't make any change. The knob itself has a terrible semantic to begin with because there is no way to express I really prefer to swap rather than page cache reclaim. This all makes me think that swappiness is a historical mistake that we should rather make obsolete than promote even further. -- Michal Hocko SUSE Labs