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

[Lennart Poettering]

On Do, 30.03.23 18:56, 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.

Linux will swap out stuff only if it has better uses for the RAM. So
yeah, apparently your VMs where mostly idle, and the RAM was better
used for other stuff, and ultimately helped speed up things for that
other more frequently used stuff. Which is an overall win, not a loss.

If the key requirement you have to make VMs migrate quickly, then
yeah, then allowing them to be written to disk is of course a
problem. But frankly, if the ability to migrate VMs quickly is your
top priority and general performance irrelevant, then you might have
weird priorities? Also, are you sure your network is faster than your
local disk?

generally though: i am not doubting that sometimes latency matters for
certain jobs, and paging stuff back in is slow and thus makes
latencies worse. But the way to address that is not turn of swap for
everything, but just for the jobs where the latency means, via the
appropriate cgroup settings. The thing is, anonymous memory is just
one kind of memory, and if you turn off swap then your force that to
remain in RAM – but at the same time you still allow file-based stuff
to be reclaimed so that it must be reread later from disk. If you use
the right resource management settings you have much better control on
that, too, and can comprehensively solve the issue, and get the
latencies you want.

Or to turn this around: if you are concerned about the latencies swap
is supposed to "introduce", but you do not run your whole OS from an
in-memory image too, then you are doing things wrong and not actually
solving what you want to solve.

Lennart

--
Lennart Poettering, Berlin