[gentoo-user] Re: /var/tmp on tmpfs

[Kai Krakow <hurikhan77@gmail.com>]

Am Thu, 08 Feb 2018 14:50:31 -0500 schrieb Rich Freeman:

> On Thu, Feb 8, 2018 at 2:17 PM, Dale <rdalek1967@gmail.com> wrote:
>> As someone else pointed out, if you start using swap, that generally
>> defeats the purpose of tmpfs.
>>
>>
> I'll just add one thing to this, which I've probably already said ages
> ago:
> 
> In an ideal world swap would STILL be better than building on disk,
> because it gives the kernel fewer constraints around what gets written
> to disk.
> 
> Anything written to disk MUST end up on the disk within the dirty
> writeback time limit.  Anything written to tmpfs doesn't ever have to
> end up on disk, and if it is swapped the kernel need not do it in any
> particular timeframe.  Also, the swapfile doesn't need the same kinds of
> integrity features as a filesystem, which probably lowers the cost of
> writes somewhat (if nothing else after a reboot there is no need to run
> tmpreaper on it).
> 
> So, swapping SHOULD still be better than building on disk, because any
> object file that doesn't end up being swapped is a saved disk IO, and
> the stuff that does get swapped will hopefully get written at a more
> opportune time vs forcing the kernel to stop what is doing after 30s (by
> default) to make sure that something gets written no matter what (if it
> wasn't deleted before then).

I can only second this.

> That's all in an ideal world.  In practice I've never found the kernel
> swapping algorithms to be the best in the world, and I've seen a lot of
> situations where it hurts.  I run without a swapfile for this reason. 
> It pains me to do it because I can think of a bunch of reasons why this
> shouldn't help, and yet for whatever reason it does.

I really prefer having inactive things being swapped out than discarding 
cache from memory. But since kernel 4.9 this no longer works so well. I'm 
still seeking the reason. But for that reason, building in tmpfs is no 
longer such an appealing option as before.

Otherwise, I was quite happy with swap behavior, exactly for the reasons 
you initially outlined.


-- 
Regards,
Kai

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