This document describes common reasons for workqueue "back up" seen on Cisco Email Security Appliance (ESA).
There can be many different causes for the system workqueue to back up.
By far the most common cause of a workqueue backup is that the Cisco Email Security Appliance is receiving messages faster than its subsystems can process and deliver them.
Another cause of workqueue backup can be inordinately expensive messages. Although the system uses threaded processing, if for example an Anti-Virus
process operates upon a very large or complex archive, the CPU and RAM usage expended can slow the entire system down, reducing the overall throughput drastically. This same paradigm applies to Anti-Spam, Message Filters, Content-Filters, Footer-stamping, and Virus Outbreak Filters.
Another fairly common cause of workqueue backup is demand on the system at the network level. If the Cisco Email Security Appliance becomes very busy processing incoming connections, resources such as CPU and RAM are not as readily available for processing messages accepted onto the system, again drastically reducing throughput.