[systemd-devel] how to let systemd hibernate start/stop the swap area?

[Luca Boccassi]

On Thu, 30 Mar 2023 at 11:09, Michael Chapman <mike at very.puzzling.org> wrote:
>
> On Thu, 30 Mar 2023, Luca Boccassi wrote:
> > On Thu, 30 Mar 2023 at 10:15, Michael Chapman <mike at very.puzzling.org> wrote:
> > >
> > > On Thu, 30 Mar 2023, Lennart Poettering wrote:
> > > > On Mi, 29.03.23 13:53, Christoph Anton Mitterer (calestyo at scientia.org) wrote:
> > > >
> > > > > > > That's a bad idea btw. I'd advise you not to do that: on modern
> > > > > > > systems you want swap, since it makes anonymous memory reclaimable.
> > > > > > > I
> > > > > > > am not sure where you are getting this idea from that swap was
> > > > > > > bad.
> > > > >
> > > > > Well I haven't said it's bad, but I guess it depends on the use case
> > > > > any available RAM.
> > > >
> > > > In almost all scenarios you want swap, regardless if little RAM or a
> > > > lot. For specialist cases where you run everything from memory, and
> > > > not even programs are backed by disk there might be exceptions. But
> > > > that#s almost never the case.
> > >
> > > One specific case where I deliberately chose _not_ to use swap: large
> > > hypervisors with local storage.
> > >
> > > With swap on the host enabled, all that ended up happening was that local
> > > IO activity caused idle guest memory to be gradually swapped out.
> > > Eventually all of the swap space filled up, and the system was exactly
> > > where it would have been had it not had any swap space configured in the
> > > first place -- except that it was now _a lot_ slower to migrate those
> > > swapped-out guests to other hypervisors.
> > >
> > > - Michael
> >
> > The solution there is to ensure the cgroup configuration for the
> > slices where the guests run have memory.swap.max=0, rather than
> > disabling it for the whole system.
>
> Perhaps, but given the rest of processes on the system need just a few
> hundred MB max, and the server has hundreds of GB of RAM, it really makes
> little difference. Turning off swap altogether is plain _simpler_.

No, it does not make "little difference", there are entire subsystems
which are much worse off, if not completely useless, without swap.
Post-cgroupsv2 memory controller things are considerably different on
this front, and old "common wisdom" no longer applies.