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List:       fedora-devel-list
Subject:    Re: Enable EarlyOOM on Fedora KDE - Fedora 33 Self-Contained Change proposal
From:       Benjamin Berg <bberg () redhat ! com>
Date:       2020-07-20 9:54:58
Message-ID: 50cf3b3eb662ec8083989a1d7fa120c80cb2c60e.camel () redhat ! com
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On Mon, 2020-07-20 at 10:55 +0200, Kevin Kofler wrote:
> That said, I do not see how the EarlyOOM heuristic, which allows, depending 
> on the exact settings, something like 80-90% of swap to be used IN ADDITION 
> to 90+% RAM (and will only start doing anything if BOTH RAM and swap are 
> full) can prevent thrashing in any reliable way. My thrashing scenarios have 
> had much less swap than that used. (I have twice as much swap than RAM, so 
> when the EarlyOOM heuristics trigger, my programs are already trying to use 
> almost 3 times as much RAM as is actually available!)

Yeah, I think that EarlyOOM will mostly help in the scenario where you
end up not having enough file caches to run your processes. I do
believe that this is a common scenario where the machine continues to
thrash for long periods[1].

In most cases where plenty of swap is available, the symptoms should be
a lot less sever. i.e. I would expect it to become really sluggish, but
things *should* recover much more quickly. Though, e.g. a fork-bomb
could easily cause something like that.

Anyway, I think to resolve it, we really need to enable the CPU and IO
cgroup controllers. CPU is easy (you just need to set a few properties
in systemd), but there are some road blocks to get the IO controller
working.

Also, we can probably fix systemd-logind hanging simply by assigning a
MemoryLow= allocation of e.g. 60M to system.slice and also setting
either DisableControllers=memory (create a large pool of memory for all
system services) or DefaultMemoryLow=X (delegate memory further to each
service separately).

Benjamin

[1] To be honest, I am wondering if some process might have actually
filled up all the RAM even in your case. It can be hard to tell later
on what exactly happened.

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