Question

I have a basic chatbox module on my website which is updating the chatbox every 3 seconds with Jquery Ajax call, plus another jquery call for sending new message. Nothing fancy. Normally each jquery ajax request takes only 0.2seconds. But on the server when 20+ people is connected server CPU goes very high and each ajax request starts to take 12-14 seconds, which is not acceptable. And also each httpd process consumes 3% – 4% CPU.

Update chatbox code:

updatechatbox = $.getJSON("/chat/update",{ lastid: $("#messageBox li:last-child").attr("id") }, function(json) {
    $.each(json, function(key, val) {
        var m = val['message'];
        var id = val['id'];
        var messagebox = $("#messageBox ul");
        messagebox.append("<li id="+id+"><span class='msg'>"+m+"</span></li>");

        var myDiv = $("#messageBox");
        myDiv.animate({ scrollTop: myDiv.prop("scrollHeight") - myDiv.height() }, 0);
    });
});

Server is Fedora 15 running nginx as proxy and apache for web service with following configuration:

Timeout 120
KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 15

IfModule prefork.c
  StartServers       8
  MinSpareServers    5
  MaxSpareServers   20
  ServerLimit      256
  MaxClients       256
  MaxRequestsPerChild  400
IfModule

IfModule worker.c
  StartServers         2
  MaxClients         150
  MinSpareThreads     25
  MaxSpareThreads     75
  ThreadsPerChild     25
  MaxRequestsPerChild  0
IfModule

Questions:
– Is this normal that apache using that much CPU for each httpd process?
– How can i fix this?

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
20089 apache    20   0 57524  16m 5840 S  7.3  0.8   0:18.16 httpd
19715 apache    20   0 56828  16m 5852 S  6.6  0.8   0:21.58 httpd
19749 apache    20   0 58536  16m 5864 S  6.6  0.8   0:22.29 httpd
19732 apache    20   0 62880  21m 5856 S  5.6  1.0   0:19.14 httpd
19803 apache    20   0 62076  21m 5840 S  5.3  1.1   0:17.94 httpd
19821 apache    20   0 61856  21m 5828 S  5.0  1.0   0:17.81 httpd
21574 apache    20   0 61584  18m 4664 S  3.3  0.9   0:00.69 httpd
19772 apache    20   0 61856  21m 5864 S  2.6  1.1   0:18.53 httpd
19932 apache    20   0 61856  20m 5844 S  2.6  1.0   0:17.07 httpd
14307 mysql     20   0  306m  52m 4576 S  2.3  2.6  81:32.57 mysqld
13175 nginx     20   0 15532 2284 1032 S  0.3  0.1   0:04.61 nginx

Edit: My PHP update function. Using Zend Framework v1.11.11

public function updateAction()
{
    $auth = Zend_Auth::getInstance();
    $user_info = $auth->getStorage()->read();
    $adminsess = new Zend_Session_Namespace("admin");
    $referrer = $_SERVER['HTTP_REFERER'];

    $user_id = $user_info->id;

    $query = "SELECT m.id, m.message, m.user_id
              FROM messagebox m
              LEFT JOIN users u ON m.user_id = u.id";

    if(isset($_GET['lastid']) && $_GET['lastid'] != ""){
        $lastId = $_GET['lastid'];
        $query .= "WHERE m.id > $lastId";
    }

    $r = $db->query($query);
    $result = $r->fetchAll();

    $data = array();

    if(count($result) > 0) {
        foreach($result as $row) {
            $data[$row['id']]['id'] = $row['id'];
            $data[$row['id']]['message'] = $row['message'];
            $data[$row['id']]['user_id'] = $user_id;
        }
    }

    echo json_encode($data);
    $this->_helper->layout->disableLayout();
}

Answer 1

I have had a similar problem before and it turned out to be to do with session locking. When a session is opened by the PHP script that starts the session this session file is locked. What this means is that if your web page makes numerous requests to PHP scripts, for instance, for loading content via Ajax, each request could be locking the session and preventing the other requests from completing. This is described in detail here.

Therefore you will need to do:

session_write_close();

If this isn’t the cause check you are not doing anything computationally intensive like, big SQL calls lots of recursion etc.