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Re: Swap priority in Debian




	Hi.

On Sat, Aug 25, 2018 at 09:42:31AM +0530, Subhadip Ghosh wrote:
> Hi,
> 
> I am a Debian testing user. Recently I am experiencing freezing on my Debian
> system intermittently and during troubleshooting the same, I found out that
> the I have a swap partition with priority set to -2.

Same here with stable.


> But according to the below manpage:
> 
> https://manpages.debian.org/stretch/mount/swapon.8.en.html
> 
> swap priority should be between -1 and 32767.

So swapon(8) utility has this restriction. Note that the system call
itself - swapon(2) merely interprets swap priority as a signed integer,
so any priority number is actually possible (within integer limits of
course).


> I have a swappiness value of 60 but I don't remember seeing the swap
> being used at all recently, the used swap is always 0%.

A hint. They invented this wonderful thing called sysstat decades ago so
you don't have to remember your swap usage, along with other things
sysstat gathers.


> My question is, do you think that the -2 priority is stopping the swap
> partition from actually being used and because of that, the system is
> getting frozen when the memory usage is high?

No, it definitely does not work this way.
A swap priority only comes into play once you have multiple instances of
swaps. If you have a single swap partition/lv/file, a priority value is
meaningless.

A system freezes, on the other hand (did I mention sysstat?), could
indicate heavy swapping, barrier writes, kernel bugs (12309, anyone?),
oom killer invocations, overheating and many other things.
A good starting point would be kernel message log (aka
/var/log/kern.log) from the time of this freeze.

Reco