Re: Postgre Eating Up Too Much RAM

Am 14.11.2012 04:19, schrieb Tom Lane:
> Craig Ringer <craig(at)2ndQuadrant(dot)com> writes:
>> On 11/14/2012 06:12 AM, Aaron Bono wrote:
>>> Am I reading this right? Are there individual connections using over
>>> 300 MB or RAM by themselves?
>> If I recall correctly, RSS is charged against a PostgreSQL back-end when
>> it touches `shared_buffers`. So that doesn't necessarily mean that the
>> back-end is using the full amount of memory listed as RSS.
> Yeah. Since Aaron's got shared_buffers set to 256MB, the shared memory
> segment is something more than that (maybe 270-280MB, hard to be sure
> without checking). The RSS numbers probably count all or nearly all of
> that for each process, but of course there's really only one copy of the
> shared memory segment. RSS is likely double-counting the postgres
> executable as well, which means that the actual additional memory used
> per process is probably just a few meg, which is in line with most
> folks' experience with PG.
>
> The "free" stats didn't look like a machine under any sort of memory
> pressure --- there's zero swap usage, and nearly half of real RAM is
> being used for disk cache, which means the kernel can find no better
> use for it than caching copies of disk files. Plus there's still 10G
> that's totally free. Maybe things get worse when the machine's been up
> longer, but this sure isn't evidence of trouble.
Keep in mind though that (SysV) SHM is accounted as "cached" in all
Linux tools (I know), thus "free" is never "complete" without "ipcs -m"
+ "ipcs -mu" outputs. However I second Tom here; your machine looks
perfectly healthy.
Note that RSS usage of your sessions can quickly "explode" though (due
to copy on write) if your clients start creating large return sets (and
in the worst case, take a lot of time to "collect" them).

You might consider deploying atop (atoptool.nl), which offers to sum up
all data based on user and/or process name, and will enable you to track
the usage stats to the past. Plus the latest version could already have
a seperate display for SHM usage (at least Gerlof promised me to add it ;-).

Cheers,

--
Gunnar "Nick" Bluth
RHCE/SCLA

Mobil +49 172 8853339
Email: gunnar(dot)bluth(at)pro-open(dot)de
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