[ale] Kernel 2.6 : worst-case scenarios if swap full

Jerry Yu jjj863 at gmail.com
Mon Jun 30 10:48:43 EDT 2008


So, are we saying kernel 2.6 is smart enough not to page out idle
pages allcated for Samba (no prod purpose. Example only) in
anticipation of better of thses pages by running or new programs,
merely since no swap is available?
What happens when kernel decisdes to page out these idle pages when
the swap is full?
It is an enterprise-grade RDBMS, with mem preallocated, similar to oracle's SGA.
This question was initially raised so to make an informed decision
whether remedial measures need to be considered or applied. The latter
include adding swap and down the swappiness (even to 0).


On 6/29/08, Jeff Lightner <jlightner at water.com> wrote:
> Maybe but does Linux not do virtual memory pool of both swap and
> physical memory?   On UNIX such as HP-UX there is value in having larger
> swap simply because it allows a larger preallocation of virtual memory
> by processes thereby allowing more to be started at the same time.
> This preallocation is for memory that is never really used for the most
> part.
>
>
>
> The key in the OP though was DB and that is a little different.  It
> doesn't say what DB.  I haven't done anything with MySQL but Oracle uses
> its own SGA which is in Shared Memory for the most part.   Swap really
> isn't the issue for DB.  Your DB ought to be using space that isn't
> using buffer cache either because it and the SGA's own caching mechanism
> interfere with each other.   Since I've only done Oracle on OCFS (Oracle
> Cluster Filesystem) or ASM raw devices I haven't investigated disabling
> buffer cache for ext3 or other filesystems.  It is something we
> routinely do for our Veritas filesystems on HP-UX.
>
>
>
> ________________________________
>
> From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Jim
> Kinney
> Sent: Saturday, June 28, 2008 1:09 PM
> To: ale at ale.org
> Subject: Re: [ale] Kernel 2.6 : worst-case scenarios if swap full
>
>
>
>
>
> On Sat, Jun 28, 2008 at 10:42 AM, Ed L. Cashin <ecashin at noserose.net>
> wrote:
>
> 2008/6/28 Jim Kinney <jim.kinney at gmail.com>:
> ...
>
>> It's pretty easy to add more swap space. Anthing over 2G is pretty
> useless
>> though.
>
> I think there was a time when the kernel could not use more
> than 2 GB from a swap partition, but unless there has been
> a regression, I would expect today's kernels to use large swap
> areas without trouble.
>
>
> It is not a problem for the kernel to TB's of swap. It simply becomes
> useless due to access speeds after about 2GB.
>
> 	
> 	
> 	If there's a limit, though, I'd like to know about it.  So any
> 	references where I could read about it would be appreciated.
> 	
> 	--
> 	 Ed Cashin <ecashin at noserose.net>
>
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>
>
> --
> --
> James P. Kinney III
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