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Old 03-22-2009, 02:26 AM
Mike McGrath
 
Default Memory issue

What am I missing here? This happened on db3 again -

total used free shared buffers cached
Mem: 18480632 18383512 97120 0 25968 17305332
-/+ buffers/cache: 1052212 17428420
Swap: 3145720 3145720 0


1) This box was swapping out like crazy.

2) All swap was full

3) oomkiller didn't start

4) Load went up a little bit, topped out at 3

5) no issues with the box at all, has postgres on it, kept running, wasn't
slow, box was completely responsive.

6) It's done this before, and aside from nagios alerting us the swap was
full, not had any issues with the box.

7) vm.swappiness = 60 which IIRC is the default

-Mike

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Old 03-22-2009, 04:11 AM
Matt Domsch
 
Default Memory issue

On Sat, Mar 21, 2009 at 10:26:21PM -0500, Mike McGrath wrote:
> What am I missing here? This happened on db3 again -
>
> total used free shared buffers cached
> Mem: 18480632 18383512 97120 0 25968 17305332
> -/+ buffers/cache: 1052212 17428420
> Swap: 3145720 3145720 0
>
>
> 1) This box was swapping out like crazy.
>
> 2) All swap was full
>
> 3) oomkiller didn't start
>
> 4) Load went up a little bit, topped out at 3
>
> 5) no issues with the box at all, has postgres on it, kept running, wasn't
> slow, box was completely responsive.
>
> 6) It's done this before, and aside from nagios alerting us the swap was
> full, not had any issues with the box.

There are ~30 postgres processes running, many with RSS > 1.6GB, and
nearly all marked idle. If these processes aren't actively running,
they each can get swapped out to free memory for use by the page
cache. The page cache is consuming 17GB, which it's free to do if no
applications are actually _running_ that need more memory. The
processes that are running and touching the database will cause those
disk accesses to get cached in the page cache.

--
Matt Domsch
Linux Technology Strategist, Dell Office of the CTO
linux.dell.com & www.dell.com/linux

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Old 03-22-2009, 02:52 PM
Mike McGrath
 
Default Memory issue

On Sun, 22 Mar 2009, Matt Domsch wrote:

> On Sat, Mar 21, 2009 at 10:26:21PM -0500, Mike McGrath wrote:
> > What am I missing here? This happened on db3 again -
> >
> > total used free shared buffers cached
> > Mem: 18480632 18383512 97120 0 25968 17305332
> > -/+ buffers/cache: 1052212 17428420
> > Swap: 3145720 3145720 0
> >
> >
> > 1) This box was swapping out like crazy.
> >
> > 2) All swap was full
> >
> > 3) oomkiller didn't start
> >
> > 4) Load went up a little bit, topped out at 3
> >
> > 5) no issues with the box at all, has postgres on it, kept running, wasn't
> > slow, box was completely responsive.
> >
> > 6) It's done this before, and aside from nagios alerting us the swap was
> > full, not had any issues with the box.
>
> There are ~30 postgres processes running, many with RSS > 1.6GB, and
> nearly all marked idle. If these processes aren't actively running,
> they each can get swapped out to free memory for use by the page
> cache. The page cache is consuming 17GB, which it's free to do if no
> applications are actually _running_ that need more memory. The
> processes that are running and touching the database will cause those
> disk accesses to get cached in the page cache.
>

Very interesting, I didn't realize that RSS could be swapped out for page
cache on apps that weren't in the run state.

-Mike

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