[CentOS] Effectiveness of CentOS vm.swappiness

Fri Jun 5 10:29:04 UTC 2015
Markus "Shorty" Uckelmann <shorty at koeln.de>

Am 05.06.2015 um 00:23 schrieb Dennis Jacobfeuerborn:
> If I'd have to venture a guess then I'd say there are memory pages that
> are never touched by any processes and as a result the algorithm has
> decided that it's more effective to swap out these pages to disk and use
> the freed ram for the page-cache.

That's my guess too.

> impact. If however these numbers are consistently larger than 0 then
> then that means the system is under acute memory pressure and has to
> constantly move pages between ram and disk and that will have a large
> negative performance impact on the system. This is the moment when swap
> usage becomes bad.

Gladly I don't have constant paging on all systems. And if there is
paging activity it's very low. AFAIK it's, as you already suggested,
just that some (probably unused) parts are swapped out. But, some of
those parts are the salt-minion, php-fpm or mysqld. All services which
are important for us and which suffer badly from being swapped out. I
already made some tests with swappiness 10 which mildly made it better.
But there still was swap usage. So I tend to set swappiness to 1. Which
I don't like to do, since those default values aren't there for nothing.

Is it possible that this happens because the servers are VMs on an
ESX-server. How could that affect this? How can I further debug this
problem and find out what's the culprit? I will go back to our metrics
and see if I can find any patterns/correlations.

Cheers, Shorty