process "Killed" cron email

I have been getting these emails, not a lot, a few each day.. and only from one site.

email subject:  Cron <mysite@server> cd /home/mysite/public_html/periodic; /usr/local/bin/php -q cron.php‏

email message:  /bin/sh: line 1: 22953 Killed                  /usr/local/bin/php -q cron.php

The number being displayed is the process number the cron job was running as. This means that for whatever reason, the cron job's process was killed by the server. This can be due to either running for too long, or using too many resources. You should ask your web hosting provider for more information, and possibly why it's happening.

thanks Magnussoft..