[CentOS] Effectiveness of CentOS vm.swappiness

[Dennis Jacobfeuerborn]

On 06.06.2015 04:48, Dennis Jacobfeuerborn wrote:
> On 05.06.2015 19:47, Greg Lindahl wrote:
>> On Fri, Jun 05, 2015 at 09:33:11AM -0700, Gordon Messmer wrote:
>>> On 06/05/2015 03:29 AM, Markus "Shorty" Uckelmann wrote:
>>>> 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.
>>>
>>> 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.
>>
>> No.
>>
>> Let's say the application only uses the page once per hour. If there
>> is also I/O going on, then it's easy to see that the kernel could
>> decide to page the page out after 50 minutes, leaving the application
>> having to page it back in 10 minutes later.
> 
> That's true but it also means that if you lock that page so it cannot be
> swapped out then this page is not available for the page cache so you
> incur the i/o hit either way and it's probably going to be worse because
> the system has no longer an option to optimize the memory management.
> I wouldn't worry about it until there's actually permanent swap activity
> going on and then you have to decide if you want to add more ram to the
> system or maybe find a way to tell e.g. Bacula to use direct i/o and not
> pollute the page cache.
> For application that do not allow to specify this a wrapper could be
> used such as this one:
> http://arighi.blogspot.de/2007/04/how-to-bypass-buffer-cache-in-linux.html

Actually I found better links:
https://code.google.com/p/pagecache-mangagement/
http://lwn.net/Articles/224653/

"It is to address the "waah, backups fill my memory with pagecache" and
the "waah, updatedb swapped everything out" and the "waah, copying a DVD
gobbled all my memory" problems."

Regards,
  Dennis