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Date:   Mon, 21 Sep 2020 18:27:21 +0200
From:   Andrea Righi <andrea.righi@canonical.com>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Luigi Semenzato <semenzato@google.com>,
        Pavel Machek <pavel@ucw.cz>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Len Brown <len.brown@intel.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>
Subject: Re: [RFC PATCH 2/2] PM: hibernate: introduce opportunistic memory
 reclaim
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On Mon, Sep 21, 2020 at 05:36:30PM +0200, Rafael J. Wysocki wrote:
...
> > > 3. It is not clear how much mm_reclaim/release is going to help.  If
> > > the preloading of the swapped-out pages uses some kind of LIFO order,
> > > and can batch multiple pages, then it might help.  Otherwise demand
> > > paging is likely to be more effective.  If the preloading does indeed
> > > help, it may be useful to explain why in the commit message.
> >
> > Swap readahead helps a lot in terms of performance if we preload all at
> > once. But I agree that for the majority of cases on-demand paging just
> > works fine.
> >
> > My specific use-case for mm_reclaim/release is to make sure a VM
> > that is just resumed is immediately "fast" by preloading the swapped-out
> > pages back to memory all at once.
> >
> > Without mm_reclaim/release I've been using the trick of running swapoff
> > followed by a swapon to force all the pages back to memory, but it's
> > kinda ugly and I was looking for a better way to do this. I've been
> > trying also the ptrace() + reading all the VMAs via /proc/pid/mem, it
> > works, but it's not as fast as swapoff+swapon or mm_reclaim/release.
> >
> > I'll report performance numbers of mm_reclaim/release vs ptrace() +
> > /proc/pid/mem in the next version of this patch.
> 
> Sorry for the huge delay.
> 
> I'm wondering what your vision regarding the use of this mechanism in
> practice is?
> 
> In the "Testing" part of the changelog you say that "in the
> 5.7-mm_reclaim case a user-space daemon detects when the system is
> idle and triggers the opportunistic memory reclaim via
> /sys/power/mm_reclaim/run", but this may not be entirely practical,
> because hibernation is not triggered every time the system is idle.
> 
> In particular, how much time is required for the opportunistic reclaim
> to run before hibernation so as to make a significant difference?
> 
> Thanks!

Hi Raphael,

the typical use-case for this feature is to hibernate "spot" cloud
instances (low-priority instances that can be stopped at any time to
prioritize more privileged instances, see for example [1]). In this
scenario hibernation can be used as a "nicer" way to stop low priority
instances, instead of shutting them down.

Opportunistic memory reclaim doesn't really reduce the time to hibernate
overall: performance wise regular hibernation and hibernation w/
opportunistic reclaim require pretty much the same time.

But the advantage of opportunistic reclaim is that we can "prepare" a
system for hibernation using some idle time, so when we really need to
hibernate a low priority instance, because a high priority instance
requires to run, hibernation can be significantly faster.

What do you think about it? Do you see a better way to achieve this
goal?

Thanks,
-Andrea

[1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-interruptions.html