PostgreSQL and cgroup

Hello:

Sorry for disturbing,
In order to make my question clear,
I wrote this one as a seperate question.

If using cgroup, I can find wget work well.
But , for postgresql, when I deal huge amount of data, it still report out
of memory error. In fact I hope postgresql can work under a limit amount of
memory.

My test is as this:
Step 1:To make memory limit through cgroup:
[postgres(at)cent6 Desktop]$ cat /etc/cgconfig.conf
...
mount {
cpuset = /cgroup/cpuset;
cpu = /cgroup/cpu;
cpuacct = /cgroup/cpuacct;
memory = /cgroup/memory;
devices = /cgroup/devices;
freezer = /cgroup/freezer;
net_cls = /cgroup/net_cls;
blkio = /cgroup/blkio;
}

group test1 {
perm {
task{
uid=postgres;
gid=postgres;
}

admin{
uid=root;
gid=root;
}

} memory {
memory.limit_in_bytes=30M;
}
}

[postgres(at)cent6 Desktop]$
[postgres(at)cent6 Desktop]$ cat /etc/cgrules.conf
# /etc/cgrules.conf
...
#<user> <controllers> <destination>
postgres memory test1/
#
[postgres(at)cent6 Desktop]$
[postgres(at)cent6 Desktop]$ chkconfig --list cgconfig
cgconfig 0:off 1:off 2:on 3:on 4:on 5:on 6:off
[postgres(at)cent6 Desktop]$ chkconfig --list cgred
cgred 0:off 1:off 2:on 3:on 4:on 5:on 6:off
[postgres(at)cent6 Desktop]$

Step 2 :To see memory :
[postgres(at)cent6 Desktop]$ free -m
total used free shared buffers cached
Mem: 2006 384 1622 0 26 138
-/+ buffers/cache: 219 1787
Swap: 4031 87 3944
[postgres(at)cent6 Desktop]$

Step3: To start postgresql and deal with data:
postgres=# select count(*) from test01;
count
-------
0
(1 row)

postgres=# insert into test01 values(generate_series(1,614400),repeat(
chr(int4(random()*26)+65),1024));

Then, In psql client, I got:
The connection to the server was lost. Attempting reset: Failed.
!>

In postgresql's log, I found:
[postgres(at)cent6 pgsql]$ LOG: database system was shut down at 2013-09-09
16:20:29 CST
LOG: database system is ready to accept connections
LOG: autovacuum launcher started
LOG: server process (PID 2697) was terminated by signal 9: Killed
DETAIL: Failed process was running: insert into test01
values(generate_series(1,614400),repeat( chr(int4(random()*26)+65),1024));
LOG: terminating any other active server processes
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the
current transaction and exit, because another server process exited
abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and
repeat your command.
FATAL: the database system is in recovery mode
LOG: all server processes terminated; reinitializing
LOG: database system was interrupted; last known up at 2013-09-09 17:35:42
CST
LOG: database system was not properly shut down; automatic recovery in
progress
LOG: redo starts at 1/9E807C90
LOG: unexpected pageaddr 1/946BE000 in log file 1, segment 159, offset
7069696
LOG: redo done at 1/9F6BDB50
LOG: database system is ready to accept connections
LOG: autovacuum launcher started

When I see dmesg, I can see:

[postgres(at)cent6 Desktop]$ dmesg | grep post
[ 2673] 500 2673 64453 200 0 0 0 postgres
[ 2675] 500 2675 64494 79 0 0 0 postgres
[ 2676] 500 2676 64453 75 0 0 0 postgres
[ 2677] 500 2677 64453 77 0 0 0 postgres
[ 2678] 500 2678 64667 80 0 0 0 postgres
[ 2679] 500 2679 28359 72 0 0 0 postgres
[ 2697] 500 2697 64764 100 0 0 0 postgres
[ 2673] 500 2673 64453 200 0 0 0 postgres
[ 2675] 500 2675 64494 79 0 0 0 postgres
[ 2676] 500 2676 64453 75 0 0 0 postgres
[ 2677] 500 2677 64453 77 0 0 0 postgres
[ 2678] 500 2678 64667 80 0 0 0 postgres
[ 2679] 500 2679 28359 72 0 0 0 postgres
[ 2697] 500 2697 64764 100 0 0 0 postgres
[ 2673] 500 2673 64453 208 0 0 0 postgres
[ 2675] 500 2675 64494 79 0 0 0 postgres
[ 2676] 500 2676 64453 98 0 0 0 postgres
[ 2677] 500 2677 64453 782 0 0 0 postgres
[ 2678] 500 2678 64667 133 0 0 0 postgres
[ 2679] 500 2679 28359 86 0 0 0 postgres
[ 2697] 500 2697 73075 3036 0 0 0 postgres
Memory cgroup out of memory: Kill process 2697 (postgres) score 1000 or
sacrifice child
Killed process 2697, UID 500, (postgres) total-vm:292300kB,
anon-rss:8432kB, file-rss:3712kB
[postgres(at)cent6 Desktop]$

Although I encountered the out of memory , but it is cgroup out of memory.
I think that cgroup really influenced postgresql.
But not by the means I hope: I hope PG can work under limit , not to be
killed by OOM.

My above sql statement quickly failed. that is: PG 's main process is
killed by OOM because it is going to use memory more than 30M,which is
limited by cgroup.

But If I run wget, wget can work well under memory limit:
such as wget
http://centos.arcticnetwork.ca/6.4/isos/x86_64/CentOS-6.4-x86_64-LiveCD.iso

So My question is:
Wen under cgroup's memory limit , Why PG crashed but wget do not?

Best Regards