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

[Christoph Anton Mitterer <calestyo@xxxxxxxxxxxx>]

Hey Lennart.


On Wed, 2023-03-29 at 16:35 +0200, Lennart Poettering wrote:
> 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.

Similar to the latter example of yours, one could think of scenarios
with little disk space, where it might be interesting to use a swap
file for hibernation, but have the storage available when the system is
up.


> But
> that#s almost never the case.

IMO that's always hard to say... in WLCG we actually have compute nodes
with little to no disk space but plenty of RAM (or well at least a
certain amount of GB per core). Though we don't need hibernation there.


> It allows the kernel to reclaim anonymous memory, because it can
> write
> it to disk and then use it for other purposes.

Well that's clear, it's just that on my systems (both servers and
workstations) I've never really run into the need to reclaim lots of
anonymous memory.
It was apparently always enough to have cached data reclaimed.


> swap is not an "extra
> on top", that's a complete misunderstanding how modern memory
> management works. By avoiding swap you create artificial
> (i.e. unnecessary) scarcity, and disallow the kernel to use RAM for
> useful purposes because you block it with anonymous memory that might
> never be used. You artificially amplify IO on the file-backed pages
> hence, because those become the only ones that are reclaimable.

Well, I have been operating like that for many years now, and back then
I actually run into trashing more often, so maybe my experiences are
just outdated :D

If things changed, I'll happily try again to run with swap and see how
it turns out for my use cases :-)


> https://chrisdown.name/2018/01/02/in-defence-of-swap.html

Thanks for the pointer.





On Wed, 2023-03-29 at 16:36 +0200, Lennart Poettering wrote:
> Yeah, all requests that go through logind check that.
> 
> You can override the check via an environment variable,
> SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1 btw, see
> https://systemd.io/ENVIRONMENT/

Nice, that did the trick.
Perfect that it's an env var that takes effect at logind, so one can
easily add it to the env of just that via the unit file, without the
need to fiddle around with bashrc and friends.


Since you're anyway rather against the whole idea,... I assume you
wouldn't want a PR that adds something like that as an example to the
manpages?
Or perhaps somehow a hint that logind is consulted first and thus the
swap area needs to be already there or the check disabled?

Another thing that could be worth to add somewhere is where people are
intended to add other service dependencies for things that should be
done before/after suspend/resume/etc. ... i.e. the respective .target
and not .service.
Might make it easier for people to use it properly :-)


In any case, thanks for your help :-)

Cheers,
Chris.