linux-kernel - Re: [PATCH] mm/vmscan: add vm_swappiness configuration knobs

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Date:   Thu, 12 Mar 2020 10:25:31 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Ivan Teterevkov <ivan.teterevkov@...anix.com>
Cc:     "corbet@....net" <corbet@....net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mchehab+samsung@...nel.org" <mchehab+samsung@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "pawan.kumar.gupta@...ux.intel.com" 
        <pawan.kumar.gupta@...ux.intel.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "oneukum@...e.com" <oneukum@...e.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/vmscan: add vm_swappiness configuration knobs

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