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

[Lennart Poettering]

On Do, 30.03.23 13:16, Phillip Susi (phill at thesusis.net) wrote:

>
> Lennart Poettering <lennart at poettering.net> writes:
>
> > oomd/PSI looks at memory allocation latencies to determine memory
> > pressure. Since you disallow anonymous memory to be paged out and thus
> > increase IO on file backed memory you increase the latencies
> > unnecessarily, thus making oomd trigger earlier.
>
> Did this get changed in the last few years?  Because I'm sure it used to
> be based on the total commit limit, and so OOM wouldn't start killing
> until your swap was full, which didn't happen until the system was
> thrashing itself to uselessness for 20 minutes already.

oomd becomes active on two distinct triggers:

This one:

https://github.com/systemd/systemd/blob/main/src/oom/oomd-manager.c#L383

and this one:

https://github.com/systemd/systemd/blob/main/src/oom/oomd-manager.c#L486

The latter is PSI.

> What happens if you use zswap?  Will hibernation try to save things to
> there instead of a real disk swap?  It might be nice to have zswap for
> normal use and the on disk swap for hibernate.

our sleep code does not consider zram devices for hibernation.

Lennart

--
Lennart Poettering, Berlin