TABLE OF CONTENTS You probably noticed that thus far in the book we have been relying on various types of rings and queues for communications between microengines. You probably also noticed the serious lack of details regarding the implementation of these rings and queues! Well, you re in luck because this chapter covers the hardware support for rings and queues on the IXP2XXX processor. Specifically, you ll learn about the software for initializing and accessing (enqueueing, dequeueing, checking the size, etc.) information in these data structures.
Both rings and queues implement a First-In-First-Out (FIFO) data structure. In the IXP2XXX literature, rings are fixed-sized, circular FIFOs, whereas queues are not fixed-sized FIFOs. FIFOs, whether implemented as rings or queues, are an extremely common concept in networking for several reasons:
The rates of the tasks producing and consuming on the ring or queue may not be identical. Although when the producing task is faster than the consuming task the ring or queue eventually fills. Rings and queues do insulate the two tasks from temporary bursts or stalls present in either task.
Multiple producing tasks can be coupled with a single consuming task. For example, this coupling might be used to enable multiple processing tasks to all use a single transmit task.
Using multiple rings or queues, a single producing task can be coupled with multiple consuming tasks. This coupling might be used in a system where some packets require different processing than others.
Different service preferences can be given to different rings or queues. For example,...
