diff --git a/alerts.php b/alerts.php
index af7c835061..878c23e326 100755
--- a/alerts.php
+++ b/alerts.php
@@ -25,11 +25,29 @@
 
 include("includes/defaults.inc.php");
 include("config.php");
+
+$lock = false;
+if( file_exists($config['install_dir']."/.alerts.lock") ) {
+	$pids = explode("\n", trim(`ps -e | grep php | awk '{print $1}'`));
+	$lpid = trim(file_get_contents($config['install_dir']."/.alerts.lock"));
+	if( in_array($lpid,$pids) ) {
+		$lock = true;
+	}
+}
+
+if( $lock === true ) {
+	exit('Alreading running with PID '.$lpid."\r\n");
+} else {
+	file_put_contents($config['install_dir']."/.alerts.lock", getmypid());
+}
+
 include("includes/definitions.inc.php");
 include("includes/functions.php");
 
 RunAlerts();
 
+unlink($config['install_dir']."/.alerts.lock");
+
 /**
  * Run all alerts
  * @return void
diff --git a/doc/Alerting.md b/doc/Alerting.md
index 0a721db18a..649b1eb6da 100644
--- a/doc/Alerting.md
+++ b/doc/Alerting.md
@@ -43,7 +43,8 @@ __Conditions__ can be any of:
 __Values__ can be Entities or any single-quoted data.  
 __Glues__ can be either `&&` for `AND` or `||` for `OR`.
 
-__Note__: The difference between `Equals` and `Matches` (and it's negation) is that `Equals` does a strict comparison and `Matches` allows the usage of the placeholder `*`. The placeholder `*` is comparable with `.*` in RegExp.
+__Note__: The difference between `Equals` and `Matches` (and it's negation) is that `Equals` does a strict comparison and `Matches` allows the usage of the placeholder `@`. The placeholder `@` is comparable with `.*` in RegExp.  
+Arithmetics are allowed as well.
 
 ## <a name="rules-examples">Examples</a>
 
diff --git a/librenms.cron b/librenms.cron
index f2171210ff..49f74898da 100644
--- a/librenms.cron
+++ b/librenms.cron
@@ -2,4 +2,4 @@
 */5  *    * * *   root    /opt/librenms/discovery.php -h new >> /dev/null 2>&1
 */5  *    * * *   root    /opt/librenms/poller-wrapper.py 16 >> /dev/null 2>&1
 15   0    * * *   root    sh /opt/librenms/daily.sh >> /dev/null 2>&1
-*/2  *    * * *   root    /opt/librenms/alerts.php >> /dev/null 2>&1
+*    *    * * *   root    /opt/librenms/alerts.php >> /dev/null 2>&1