Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude protection

From: Andrew Morton <akpm@linux-foundation.org>
To: Chris Down <chris@chrisdown.name>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>, Tejun Heo <tj@kernel.org>,
	Roman Gushchin <guro@fb.com>, Dennis Zhou <dennis@kernel.org>,
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
	linux-mm@kvack.org, kernel-team@fb.com
Subject: Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude protection
Date: Fri, 22 Mar 2019 13:10:15 -0700	[thread overview]
Message-ID: <20190322131015.05edf9fac014f4cacf10dd2a@linux-foundation.org> (raw)
In-Reply-To: <20190322160307.GA3316@chrisdown.name>

On Fri, 22 Mar 2019 16:03:07 +0000 Chris Down <chris@chrisdown.name> wrote:

> This patch is an incremental improvement on the existing
> memory.{low,min} relative reclaim work to base its scan pressure
> calculations on how much protection is available compared to the current
> usage, rather than how much the current usage is over some protection
> threshold.
> 
> Previously the way that memory.low protection works is that if you are
> 50% over a certain baseline, you get 50% of your normal scan pressure.
> This is certainly better than the previous cliff-edge behaviour, but it
> can be improved even further by always considering memory under the
> currently enforced protection threshold to be out of bounds. This means
> that we can set relatively low memory.low thresholds for variable or
> bursty workloads while still getting a reasonable level of protection,
> whereas with the previous version we may still trivially hit the 100%
> clamp. The previous 100% clamp is also somewhat arbitrary, whereas this
> one is more concretely based on the currently enforced protection
> threshold, which is likely easier to reason about.
> 
> There is also a subtle issue with the way that proportional reclaim
> worked previously -- it promotes having no memory.low, since it makes
> pressure higher during low reclaim. This happens because we base our
> scan pressure modulation on how far memory.current is between memory.min
> and memory.low, but if memory.low is unset, we only use the overage
> method. In most cromulent configurations, this then means that we end up
> with *more* pressure than with no memory.low at all when we're in low
> reclaim, which is not really very usable or expected.
> 
> With this patch, memory.low and memory.min affect reclaim pressure in a
> more understandable and composable way. For example, from a user
> standpoint, "protected" memory now remains untouchable from a reclaim
> aggression standpoint, and users can also have more confidence that
> bursty workloads will still receive some amount of guaranteed
> protection.

Could you please provide more description of the effect this has upon
userspace?  Preferably in real-world cases.  What problems were being
observed and how does this improve things?