Re: how to let systemd hibernate start/stop the swap area?

[Michael Chapman <mike@xxxxxxxxxxxxxxxxx>]

On Thu, 30 Mar 2023, Greg KH wrote:
> On Thu, Mar 30, 2023 at 09:09:19PM +1100, Michael Chapman wrote:
> > On Thu, 30 Mar 2023, Luca Boccassi wrote:
> > > On Thu, 30 Mar 2023 at 10:15, Michael Chapman <mike@xxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Thu, 30 Mar 2023, Lennart Poettering wrote:
> > > > > On Mi, 29.03.23 13:53, Christoph Anton Mitterer (calestyo@xxxxxxxxxxxx) 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_.
> 
> So you penalize the runtime performance of guests for the infrequent
> migration delay?  Sounds like a bad trade-off for any real workload
> those guests are doing.  Shouldn't the goal of the system be to solve
> the problem the guests are trying to solve instead of being optimized
> for the infrequent administration tasks?

Err... how is ensuring the guests actually stay in memory "penalising 
their runtime performance"? If anything, it's exactly the opposite!

Note that the hypervisor wouldn't have been overprovisioned. If it were, 
say, a 256 GB server, I might only put 200 GB of guests on it. That still 
leaves around 50 GB for page cache, which is more than enough!

Adding a few GB of swap to the system would hardly make a difference.