Re: free RAM not being used for page cache

Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
> On Wed, Jul 30, 2014 at 12:51 PM, Kevin Goess <kgoess(at)bepress(dot)com> wrote:
>
>> A couple months ago we upgraded the RAM on our database servers from 48GB to
>> 64GB.  Immediately afterwards the new RAM was being used for page cache,
>> which is what we want, but that seems to have dropped off over time, and
>> there's currently actually like 12GB of totally unused RAM.

> could be a numa issue.

I was thinking the same thing.

The other thought was that it could be a usage pattern and/or
monitoring issue.  When there are transient requests for large
amounts of memory, it will discard cache to satisfy those (e.g.,
work_mem or maintenance_work_mem allocations).  If the *active*
portion of the database is not as big as RAM, it might not refill
right away.  This could be compounded on your monitoring graphs if
they summarize by taking the *average* RAM usage for an interval
rather than the *maximum* usage for that interval.  Intermittent
spikes in usage could make it look like the RAM is unused if you
are averaging; personally, I would prefer to use maximum for a
metric like this.  Many monitoring systems allow you to choose.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company