"Allowed memory size of 536870912 bytes exhausted "问题

Allowed memory size of 536870912 bytes exhausted (tried to allocate 130968 bytes)
什么原因导致的，求大神解惑。

public function test()
{
$start = '1444320000';
$end = '1444406399';
$shop_pv_opt = $this->get_shop_pv($start, $end);
$status = $this->deal_analysis($shop_pv_opt);
}
private function deal_analysis($opt)
{
$statelog_list = $this->mongodb_stat_log_obj->aggregate($opt);
}


private function get_shop_pv($start, $end)
{

$opt = array(
array(
'$match'=>array(
'time'=>array(
'$gte'=>$start,
'$lte'=>$end))),
array(
'$group'=>array(
'_id'=>array(
'merchant_id'=>'$merchant_id',
'pro_id'=>'$pro_id'),
'pv'=>array(
'$sum'=>1))),
array(
'$sort'=>array(
'_id'=>1)));
return $opt;
}
public function aggregate($opt)
{
  $this->selectCollection('stat_log');
  $result = $this->aggregate($opt);//执行aggregate时mongo里只有十几条数据，但是报内存超限错误
  return $result;
}