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Recently, the session table on one of our client's site grew to 3.2 GB and got locked and the server eventually became full with PHP opening lots of connections trying to open the sessions table.

To resolve this issue, we found the Session Expire module (http://drupal.org/project/session_expire). This module trims the session regularly based on cron job, and hence is a background process, and is consistent and predictable regardless of PHP's garbage collection configuration.

This module is a great utility for bigger sites with large user base.