Beaucoup de requêtes dormantes s'exécutent sur le processus mysql

Nous avons 150 bases de données qui tournent sur le serveur et parfois il y a un problème de "trop de connexions" et il y a aussi beaucoup de requêtes dormantes qui tournent sur le processus MySQL. Le temps des requêtes dormantes est supérieur à 200. Vous trouverez ci-dessous la configuration my.cnf du serveur. J'ai augmenté le max_connections de 500 à 750 mais les connexions augmentent parfois au-delà de 750. Est-il sûr d'augmenter le max_connections à 1000 ou d'avoir besoin d'une configuration supplémentaire dans le my.cnf ? Je vous demande de m'aider, s'il manque quelque chose dans la configuration.

[mysqld]
performance-schema=0
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
symbolic-links=0

log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
query_cache_type=1

slow_query_log=1
slow_query_log_file="/var/log/mysql/mysql_slow.log"
max_connections = 750
default-storage-engine=MyISAM
innodb_file_per_table=1
max_allowed_packet=268435456
open_files_limit=43000
bind-address=0.0.0.0
innodb_buffer_pool_size= 6G
wait_timeout = 300
key_buffer_size = 2G
read_buffer_size = 16M
bulk_insert_buffer_size = 512M
myisam_sort_buffer_size = 2M
sql_mode = 'NO_ENGINE_SUBSTITUTION'
query_cache_limit = 536870912
query_cache_size = 268435456

Threads Running On Mysql

mysql> show status like "%Threads%";
+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| Delayed_insert_threads | 0     |
| Slow_launch_threads    | 0     |
| Threads_cached         | 12    |
| Threads_connected      | 659   |
| Threads_created        | 38451 |
| Threads_running        | 1     |
+------------------------+-------+

les résultats de l'étude htop :

1  [|      1.3%]   11 [       0.0%]   21 [       0.0%]   31 [       0.0%]
2  [||     3.2%]   12 [||||  29.5%]   22 [       0.0%]   32 [       0.0%]
3  [       0.0%]   13 [       0.0%]   23 [|      0.6%]   33 [       0.0%]
4  [|||   21.3%]   14 [||||  26.1%]   24 [||     2.6%]   34 [       0.0%]
5  [|      0.6%]   15 [|      0.6%]   25 [||     1.9%]   35 [       0.0%]
6  [||||  26.3%]   16 [||     3.2%]   26 [||     1.3%]   36 [       0.0%]
7  [||     1.9%]   17 [||     4.5%]   27 [       0.0%]   37 [       0.0%]
8  [||     3.8%]   18 [||     5.8%]   28 [       0.0%]   38 [       0.0%]
9  [|      0.6%]   19 [       0.0%]   29 [       0.0%]   39 [       0.0%]
10 [||     4.5%]   20 [||     5.1%]   30 [       0.0%]   40 [       0.0%]
Mem[|||||||||||||||||||17.7G/62.7G]   Tasks: 132, 473 thr; 3 running
Swp[|                  89.0M/7.81G]   Load average: 1.03 0.97 0.90 
                                        Uptime: 22 days, 16:31:03 

 PID USER      PRI  NI  VIRT   RES   SHR S CPU% MEM%   TIME+  Command
14706 mysql      20   0 29.3G 13.7G 10816 S 12.2 21.9 37h23:37 /usr/sbin/mysqld

Connexion à la base de données script (PHP) :

<?php
class setting{  
    private $conn;
    private $dbs;   
    function __construct($cid){
        $client=$this->getclientbyid($cid);     
        $db=new db_bridge($client['website']);
        if(!$db->exist()){
            echo($client['website']." not exist in our DB");
            die;    
        }
        else{
            $db->setid();   
            $this->setdb($db->get());
        }   
   }
   function __destruct(){
    $this->dbs=null;
   }
   function setdb($db){
    $this->conn=$db;
    self::connect();    
   }
   function connect(){
        try{
        $this->dbs=new PDO("mysql:host=".$this->conn['host'].";dbname=".$this->conn['db'], $this->conn['duser'],$pass);
        }
        catch(Exception $e){
            echo "Unable to connect Database";
        }        
    }
    protected function query($query,$both=false){
        $result=$this->dbs->prepare($query);
        $result->execute();
        if(!$both){
            $rs=$result->fetch(PDO::FETCH_ASSOC);
        }
        else{
            $rs=$result->fetch(PDO::FETCH_BOTH);
        }
        return $rs;
    }
}