On 04.06.2015 22:18, Markus "Shorty" Uckelmann wrote:
Hi all,
This might not be CentOS related at all. Sorry about that.
I have lots of C6 & C7 machines in use and all of them have the default swappiness of 60. The problem now is that a lot of those machines do swap although there is no memory pressure. I'm now thinking about lowering swappiness to 1. But I'd still like to find out why this happens. The only common thing between all those machines is that there are nightly backups done with Bacula. I once came across issues with the fs-cache bringing Linux to start paging out. Any hints, explanations and suggestions would be much appreciated.
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. Swap usage isn't inherently evil and what you really want to check for is the si/so columns in the output of the "vmstat" command. If the system is using swap space but these columns are mostly 0 then that means memory has been swapped out in the past but there is no actual swap activity happening right now and there should be no performance 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.
Regards, Dennis