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Re: Moving /tmp to tmpfs makes it useless

On 05/26/2012 09:20 AM, Nikolaus Rath wrote:
> I believe tmpfs memory is swapped out preferentially, so your scenario
> doesn't have to play out like that. However, paging being a complex
> process, it's not impossible either.

<disclaimer>I haven't read the actual kernel code, just making assumption
from what I generally know about swapping algorithms (eg: a long long time
ago, I worked on something like that on Atari computers...).</disclaimer>

What's going to go to the swap space are pages that have a lower rank.
There are many ways to rank a page. But these pages are generally
what hasn't been used often, and hasn't been used for a while. So let's
say you start an app, and don't use at all some of its functions, it gets
swapped out (if needed) until you start using it. This is where one can
complain about the randomness of swap access: it's not really predictable
from the user's point of view, while having a file written to /tmp is.

If you access often files in your tmpfs, of course, they wont go to the
swap, they will have a higher rank, and the kernel will decide to swap
some pages that are less often accessed. And that's when it becomes
very annoying, whatever is the swap algorithm: things go in and out
of the swap space depending on how often you use them, when all
your app cared was writing a (potentially) big file to disk. Here, we
don't care if things are going faster (benches are useless), we care
that it "feels" right, and we care our system is reactive to what the
user asks for.

Now, what happens if your /tmp gets full? Hum... generally, not much.
The application fails, and if written correctly, displays an error message.
If written badly, it simply crashes. In all cases, that's not a big deal.
But if your swap gets full (or used really a lot), then it's a disaster.
Your system becomes unresponsive to users commands, and sometimes
even crashes (eg: the OOM killer starts acting funnily and kills random
processes...). No way anyone wants this on his system. Even less a user
with not a lot of experience, who just happen to choose the installer's
default, and that wont understand what's happening.


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