Re: Enable EarlyOOM on Fedora KDE - Fedora 33 Self-Contained Change proposal

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On Sun, 2020-07-12 at 14:46 -0700, John M. Harris Jr wrote:
> That sounds like an excellent idea, but I'm not convinced that killing users' 
> processes, while there's still tons of memory free available, is actually an 
> improvement.

I think you should rather think of "MemAvailable" as a measure of how
flexible the kernel believes it is in its effort to do memory
management.


As I said elsewhere already. MemAvailable (i.e. the kernel measure that
is read from /proc), does *not* reliably tell you how much memory you
can use. There often is no such thing as free memory, it'll be in use
for some purpose.

And, even worse, the kernel cannot even know. Unless you are already at
the point of swapping/thrashing. This is because the kernel does not
know know which memory pages the applications are actively using. These
measure really are just a guess, rather than a reliably and hard
number.

If you actually tried to use the memory it says in MemAvailable, you
may very well already get bad side effects as the kernel needs to
reclaim memory used for other purposes (file caches, mmap'ed
executables, heap, …). Depending on the workload, this may already
cause the system to start thrashing.


I am not trying to claim that EarlyOOM is perfect. These are all just
rough heuristics and how well they work will heavily depend on the
workload. I am sure you can find scenarios where EarlyOOM will be too
aggressive.
But, I also think that the people proposing this have done quite a lot
of testing to find reasonable values for various scenarios. If they
have done their job correctly, then EarlyOOM will *not* prevent you
from fully utilizing your system memory in most scenarios.

Benjamin

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