Memory issues

Hi,
I'm using and Amazon ec2 instance with the following spec and the application that I'm running uses a postgres DB 9.1.
The app has 3 main cron jobs.

Ubuntu 12, High-Memory Extra Large Instance
17.1 GB of memory
6.5 EC2 Compute Units (2 virtual cores with 3.25 EC2 Compute Units each)
420 GB of instance storage
64-bit platform

I've changed the main default values under file postgresql.conf to:
shared_buffers = 4GB
work_mem = 4GB
wal_buffers = 16MB
checkpoint_segments = 32
effective_cache_size = 8GB

When I run the app, after an hour or two, free -m looks like below ans the crons can't run due to memory loss or similar (i'm new to postgres and db admin).
Thanks!

free -m, errors:

total used free shared buffers cached
Mem: 17079 13742 3337 0 64 11882
-/+ buffers/cache: 1796 15283
Swap: 511 0 511

total used free shared buffers cached
Mem: 17079 16833 245 0 42 14583
-/+ buffers/cache: 2207 14871
Swap: 511 0 511

**errors:
DBI connect('database=---;host=localhost','postgres',...) failed: could not fork new process for connection: Cannot allocate memory
could not fork new process for connection: Cannot allocate memory

and
execute failed: ERROR: out of memory
DETAIL: Failed on request of size 968. [for Statement "
SELECT DISTINCT....

Thank you!

A work_mem of 4GB is way, way to high.

That memory can be allocated several times by single query (and by each and every connection). Anything beyond 32MB is very uncommon.

----
Thank you, I'll check it out right away. I appreciate your help.

*Hi,
I changed to 16MB, it seems to help a bit, but still I think the queries take to much memory or don't release memory- a lot of the queries stay idle with allocated memory (I think).

What else can I check?

The free field in "free -m" just goes lower and lower and doesn't rise when nothing is running, is this ok?

Thanks!

You should take that question to the PostgreSQL mailing list. I have no experience with setting up Postgres on Linux

Thank you.