linux-kernel - Re: [PATCH v4] mm: Make memory.emin the baseline for utilisation determination

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Date:   Wed, 6 Feb 2019 15:31:17 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Roman Gushchin <guro@...com>, Dennis Zhou <dennis@...nel.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com
Subject: Re: [PATCH v4] mm: Make memory.emin the baseline for utilisation
 determination

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 <chris@...isdown.name>
> Suggested-by: Roman Gushchin <guro@...com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Roman Gushchin <guro@...com>
> Cc: Dennis Zhou <dennis@...nel.org>
> Cc: linux-kernel@...r.kernel.org
> Cc: cgroups@...r.kernel.org
> Cc: linux-mm@...ck.org
> Cc: kernel-team@...com

Acked-by: Johannes Weiner <hannes@...xchg.org>