The last two days I programmed an system with n input channels and n output channels. The system multiplex the input packet to the actual output channel, depending on packet IP destination address (and do some packet mangling, but this does not matter here). But the output channel can block (the socket returns EAGAIN, e.g. if TCP peer close the window). Each received packet is enqueued in the input queue. If the output channel is blocked then the input queue is stalled, no new packet can be transmitted, although the packet is indent for another output queue - this is bad.

The problem is also typical for switches: multiple input, multiple output and sometime the destination is the same output, thus buffering is required. The following image illustrate the layout of a Cisco Catalyst 6500. Catalyst do NOT suffer from HOL blocking because buffering is handled in egress queues - which is superior solution if the multiplex process is fast enough and do not consume to much cycles.


The idea to use a input queue was performance driven. At line rate - when CPU processing is at 100% load - I don't wanted to spend the cycles in packet processing if later on the packet is dropped with a high probability. So from a performance point of view I don't want to drop the design. But the performance issues that arise from the HOL blocking are much more relevant. So I started to look for workaraounds. The following papers look quit interesting:

RFIFO: Retreat FIFOs for the Head-of-Line Blocking problem

Head of Line blocking prevention technique based on multiple input queues per priority