psi-notify: Alerting before CPU/memory/IO becomes oversaturated
tl;dr: psi-notify can alert you when resources on your machine are becoming oversaturated, and allow you to take action before your system slows to a crawl.
I think we've all had situations where the desktop environment has become so slow as to be unusable, usually because some application has started eating up system resources quickly. Fellow Chrome users, I'm especially looking at you. :-)
My wife is certainly someone in that position. Often she gets into scenarios where unwieldy applications like Chrome would unexpectedly eat memory over time, eventually ending with her desktop suddenly slowing to a crawl. Usually the only way out of this situation is to manually trigger an OOM kill with Alt + SysRq + F, and even then that's not foolproof, since the OOM killer may not pick the right target to kill. Even if it does pick the right target to kill, system responsiveness after the fact can still be impacted since the thrashing has pushed many pages into swap.
Logging PSI metrics on her machine leading up to these incidents, it was pretty obvious that there was always an opportunity to alert her ahead of time that the situation was becoming a problem, and allow her to manually select which applications to kill, since it might not always just be the one using the most memory or with the highest memory usage in a desktop scenario. Since prioritisation of "what to kill" is so dynamic on the desktop, it's very hard to write a one-size-fits-all policy about what to do in these scenarios, so these notifications allow you to avoid that problem entirely.
To that extent, psi-notify will warn you using whatever desktop notifier you have running (eg. GNOME notifications, dunst, xfce4-notifyd) when resources are starting to become contended, so that you can do something about that. A facetious demo is shown below:
Comparison with oomd
oomd and psi-notify are two compatible and complementary projects, they're not in opposition to each other. oomd also uses PSI metrics, but it requires a policy about "what to kill" in high-pressure scenarios. For example, on a web server we obviously don't want to kill the web server if we can avoid that, so we should prioritise other applications. On the desktop though, it's hard to say: should we kill Chrome, or some IDE, or maybe something playing a movie? It's extremely difficult (although perhaps possible) to produce a single configuration that will do the right thing in even the majority of cases, so we opt to alert early instead and have the user make the decision about what's high priority in their user session. I suspect most distributions when integrating oomd for the desktop will end up having to make it less aggressive than would be ideal, so they can still interoperate.
At least in my case, if I want oomd to be aggressive, it's still hard to produce a good policy for, say, one's working day, where at one time my terminal is the most critical thing, at another my browser is, and at another it's my mail client. At other times maybe I'm ok with the slowdown and am willing to ride it out without killing anything.
Feature requests and bug reports are welcome. I've been using it for the last week and already found it useful in keeping death-by-Chrome at bay. :-)
Thanks also to Johannes for his help going over unprivileged poll() support as part of this work!