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Date:   Mon, 5 Oct 2020 15:51:30 +0200
From:   Andrea Righi <andrea.righi@canonical.com>
To:     Chris Down <chris@chrisdown.name>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Li Zefan <lizefan@huawei.com>, Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Luigi Semenzato <semenzato@google.com>,
        "Rafael J . Wysocki" <rafael@kernel.org>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH RFC v2] Opportunistic memory reclaim
Message-ID: <20201005135130.GA850459@xps-13-7390>
References: <20201005081313.732745-1-andrea.righi@canonical.com>
 <20201005112555.GA108347@chrisdown.name>
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On Mon, Oct 05, 2020 at 12:25:55PM +0100, Chris Down wrote:
> Andrea Righi writes:
> > This feature has been successfully used to improve hibernation time of
> > cloud computing instances.
> > 
> > Certain cloud providers allow to run "spot instances": low-priority
> > instances that run when there are spare resources available and can be
> > stopped at any time to prioritize other more privileged instances [2].
> > 
> > Hibernation can be used to stop these low-priority instances nicely,
> > rather than losing state when the instance is shut down. Being able to
> > quickly stop low-priority instances can be critical to provide a better
> > quality of service in the overall cloud infrastructure [1].
> > 
> > The main bottleneck of hibernation is represented by the I/O generated
> > to write all the main memory (hibernation image) to a persistent
> > storage.
> > 
> > Opportunistic memory reclaimed can be used to reduce the size of the
> > hibernation image in advance, for example if the system is idle for a
> > certain amount of time, so if an hibernation request happens, the kernel
> > has already saved most of the memory to the swap device (caches have
> > been dropped, etc.) and hibernation can complete quickly.
> 
> Hmm, why does this need to be implemented in kernelspace? We already have
> userspace shrinkers using memory pressure information as part of PID control
> already (eg. senpai). Using memory.high and pressure information looks a lot
> easier to reason about than having to choose an absolute number ahead of
> time and hoping it works.

senpai is focused at estimating the ideal memory requirements without
affecting performance. And this covers the use case about reducing
memory footprint.

In my specific use-case (hibernation) I would let the system use as much
memory as possible if it's doing any activity (reclaiming memory only
when the kernel decides that it needs to reclaim memory) and apply a
more aggressive memory reclaiming policy when the system is mostly idle.

I could probably implement this behavior adjusting memory.high
dynamically, like senpai, but I'm worried about potential sudden large
allocations that may require to respond faster at increasing
memory.high. I think the user-space triggered memory reclaim approach is
a safer solution from this perspective.

-Andrea