Re: heavy swapping, not sure why

On Mon, Aug 29, 2011 at 2:24 PM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
> On Mon, Aug 29, 2011 at 2:57 PM, Lonni J Friedman <netllama@gmail.com> wrote:
>> On Mon, Aug 29, 2011 at 1:46 PM, Alan Hodgson <ahodgson@simkin.ca> wrote:
>>> On August 29, 2011 01:36:07 PM Lonni J Friedman wrote:
>>>> I have several Linux-x68_64 based dedicated PostgreSQL servers where
>>>> I'm experiencing significant swap usage growth over time.
>>>
>>> It's the Linux kernel that does it, not PostgreSQL. Set vm.swappiness=0
>>> (usually in /etc/sysctl.conf) and put that into effect.
>>
>> I understand that the kernel determines what is swapped out, however
>> postgres is what is using nearly all the RAM, and then overflowing
>> into swap.  I guess I should have noted that this isn't a case of a
>> significant amount of RAM not being used, and swapping occurring
>> anyway.  Most of the RAM is already consumed when the heavy swapping
>> is happening.  So, I'd be surprised if setting vm.swappiness=0 will
>> make a significant difference, however I can certainly try.
>
> You haven't shown us how you determined this, it would be nice to see
> some copy and paste of things like top, free, or whatever.   How much
> free AND cache is left over when the machine starts to run out of
> memory etc.

Sorry, I was looking at the output from 'free' (plus we have some
generic monitoring tools which generate pretty graphs that also
illustrate the problem).  I restarted postgres this morning, so
everything is in a good state right now:
             total       used       free     shared    buffers     cached
Mem:         56481      55486        995          0         15      53298
-/+ buffers/cache:       2172      54309
Swap:         1099         18       1081


             total       used       free     shared    buffers     cached
Mem:        121177     111603       9573          0          0     101007
-/+ buffers/cache:      10596     110581
Swap:         1498         10       1488

Based on past results, it'll be about two weeks before a few hundred
MB of swap is in use, and perf is noticeably poor.  Although it will
creep up over time, so even in a day or 2, it will be worse than right
now.

I could post the pretty graph somewhere (or send it to the list, if
you'd prefer) if you want to see something right now (filesize is less
than 40KB).

>
> Your settings for shared_memory are HUGE.  I run a machine witih 128G
> of ram and my shared_memory is 8G and that's quite large.  No testing
> anyone has done has shown anything over 10G being useful.

Do you mean shared_buffers?

thanks