[CentOS] Effectiveness of CentOS vm.swappiness

Sat Jun 6 09:23:40 UTC 2015
Markus "Shorty" Uckelmann <shorty at koeln.de>

Am 05.06.2015 um 23:32 schrieb Gordon Messmer:
>>> Those two things can't really both be true.  If the pages
>>> swapped out are unused, then the application won't suffer as a
>>> result.
>> Why not? If you have an application which sees action only every
>> 12 to 24 hours,I think this can happen.
> Well, that's not "unused," then.

In a matter of speaking it's not "unused". But in the case of "rarely
used" it is possible that parts of the programm are in swap which are

> To measure the swap use of your processes, install "smem".  It will
> show you the amount of swap that each process is using.

Brilliant! Until now I was using the script under [1].

> For more specific information, make a copy of /proc/<pid>/smaps.
> To quantify your problem, let bacula run then save the output of
> smem, or /proc/<pid>/smaps for each of your critical services, or
> both, and then access each of the services and quantify the latency
> relative to the normal latency.  Finally, after collecting latency
> information, get the output of smem and/or /proc/<pid>/smaps again.
> You can compare swap use before and after accessing the service to
> see how much was swapped out beforehand (presumably because of the
> backup), and how much had to be recovered for your test query.
> I'd suggest collecting that information at the normal swappiness
> setting and at 0.

Thank you, this will get me further.

> If the kernel is swapping out processes in favor of filesystem
> cache when swappiness is 0, I believe that would be a bug, and
> should be reported to the kernel developers.

Because of what I read in [2] I'm not planning to use 0, rather 1. But
please correct me if I'm wrong.

> "Timeouts" is pretty vague.  Very generally, it's possible that you
> have a timeout configured somewhere that is failing on the first
> run because the filesystem cache now contains content from your
> backup, and your process only completes in time when the files
> needed for the deployment are in the filesystem cache.  That's a
> stretch as far as explanations go, but if that is the case, then
> swappiness isn't going to fix the problem.  You need to fix your
> timeout so that it allows enough time for the deployment to finish
> when the server is cold booted (using no cache), or prime your
> caches before doing deployments.

With timeouts I meant that the salt master tries to contact the
salt-minion to send it the payload. At this point happens the timeout.
In this case it means that the minion doesn't get back to the master
in the configured timeout. Currently it's set to 20 seconds. When we
start a job the first time after several hours we get a lot of
timeouts. A second run mostly helps. I think it is possible that parts
of the minion process which are needed for the payload we send it are
swapped out. At the first run it takes too long to get the pages back
into RAM. But eventually all pages are paged in. So the second run
works. But this is just an assumption. On Monday I'll try to find out
if I'm right or wrong.

BTW: Is there a way to find out which parts of a programm are swapped
out without using monsters like Valgrind? Damn, sounds like an
interesting start of the week...

[1] http://northernmost.org/blog/find-out-what-is-using-your-swap/

Cheers to all for the feedback and help,