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Comment by dbdr | Hacker News Reader

> exceed swap + a configurable amount (default is 50%) of physical RAM

Naive question: why is this default 50%, and more generally why is this not the entire RAM, what happens to the rest?

There's a lot of options. If you want to go down the rabbithole try typing `sysctl -a | grep -E "^vm"` and that'll give you a lot of things to google ;)

it's a (then-)safe default from the age when having 1GB of RAM and 2GB of swap was the norm: https://linux-kernel.vger.kernel.narkive.com/U64kKQbW/should...

Probably a safe default as there's extra memory of kernel structures, file buffering, SSH sessions to allow logins to debug why your server suddenly has high load and high IOwait (swapping).

If you know a a system is going to run (e.g.) a Postgres database, then tweaking the vm.* sysctl values is part of the tuning process.

Not sure if I understand your question but nothing "happens to the rest", overcommitting just means processes can allocate memory in excess of RAM + swap. The percentage is arbitrary, could be 50%, 100% or 1000%. Allocating additional memory is not a problem per se, it only becomes a problem when you try to actually write (and subsequently read) more than you have.

They’re talking about the never-overcommit setting.

Just a guess, but I reckon it doesn't account for things like kernel memory usage, such as caches and buffers. Assigning 100% of physical RAM to applications is probably going to have a Really Bad Outcome.

But the memory being used by the kernel has already been allocated by the kernel. So obviously that RAM isn't available.

I can understand leaving some amount free in case the kernel needs to allocate additional memory in the future, but anything near half seems like a lot!

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