Out of Memory errors although MYSQL only uses 3% of RAM

[Rea rillert]

Sorry, total noob here, and frankly I have no idea what I am doing .. so please be gentle

I have an app running on CENTOS 6 x64
24 GB RAM, 512MB swap memory

free -h shows
total used free shared buffers cached
Mem: 23G 2.3G 21G 183M 27M 275M
-/+ buffers/cache: 2.0G 21G
Swap: 512M 0B 512M

htop shows the same picture


yet mysql runs out of memory every so often

my app log shows:
[error] [exception.CDbException] SQLSTATE[HY000] [1135] Can't create a new thread (errno 11); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug
[error] [exception.CDbException] exception 'CDbException' with message 'CDbConnection failed to open the DB connection.' in /var/www/html/cededw/apps/common/framework/db/CDbConnection.php:405

I tried optimizing my settings in my.cnf

everybody told me to set innodb_buffer_pool_size to 14GB and up
This doesn't work at all
max size I can set it to is 6GB
If I try to set more I get error:
InnoDB: Initializing buffer pool, size = 20.0G
InnoDB: mmap(21978152960 bytes) failed; errno 12
InnoDB: Completed initialization of buffer pool
InnoDB: Fatal error: cannot allocate memory for the buffer pool
[ERROR] Plugin 'InnoDB' init function returned error.
[ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
[ERROR] Unknown/unsupported storage engine: InnoDB
[ERROR] Aborting


Here's my my.cnf:

[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
user=mysql
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0
innodb_file_per_table = 1
thread_concurrency = 8
query_cache_size = 0
query_cache_type = 0
query_cache_limit = 128M
thread_cache_size = 4
tmp_table_size = 160M
max_heap_table_size = 160M
myisam_sort_buffer_size = 256M
read_rnd_buffer_size = 32M
read_buffer_size = 8M
sort_buffer_size = 8M
table_open_cache = 4096
table-definition-cache = 4096
max_allowed_packet = 4M
key_buffer_size = 1336M
innodb_buffer_pool_size = 6G
open-files-limit = 65535
innodb_flush_log_at_trx_commit = 2
max_connections = 500
innodb-flush-method = O_DIRECT
innodb-log-files-in-group = 2

I googled my issues and tried almost everything I found in google

added this to /etc/security/limits.conf
* hard nofile 102400
* soft nofile 102400

Played round in my.cnf

nothing so far has worked


Can anyone tell me why I am running out of memory for mysql, what changes do I have to make to my.cnf?
or what tools to debug the issue?
Or may this be a server configuration issue?
Like server is giving priority to some other ressources?
But I still don't see why I run out of memory at all if there's 21GB of RAM untouched?


All and any help much appreciated!! Thxs