Editing Wormhole switching (section)

== Virtual channels ==

An extension of worm-hole flow control is [[Virtual-Channel]] [[flow control (data)|flow control]], where several virtual channels may be multiplexed across one physical channel. Each unidirectional virtual channel is realized by an independently managed pair of (flit) buffers. Different packets can then share the physical channel on a flit-by-flit basis. 
Virtual channels were originally introduced to avoid the deadlock problem, but they can be also used to reduce wormhole blocking, improving network latency and throughput. Wormhole blocking occurs when a packet acquires a channel, thus preventing other packets from using the channel and forcing them to stall. 
Suppose a packet P0 has acquired the channel between two routers. In absence of virtual channels, a packet P1 arriving later would be blocked until the transmission of P0 has been completed. 
If virtual channels are implemented, the following improvements are possible:

*  Upon arrival of P1, the physical channel can be multiplexed between them on a flit-by-flit basis, so that both packets proceed with half speed (depending on the arbitration scheme).
* If P0 is a full-length packet whereas P1 is only a small control packet of size of few flits, then this scheme allows P1 pass through both routers while P0 is slowed down for a short time corresponding to the transmission of few packets. This reduces latency for P1.
* Assume that P0 is temporarily blocked downstream from the current router. Throughput is increased by allowing P1 to proceed at the full speed of the physical channel. Without virtual channels, P0 would be occupying the channel, without actually using the available bandwidth (since it is being blocked).<ref>
Pavel Tvrdik.
[http://pages.cs.wisc.edu/~tvrdik/8/html/Section8.html "Why wormhole routing is an important switching technique"]
</ref>

Using virtual channels to reduce wormhole blocking has many similarities to using [[virtual output queueing]] to reduce [[head-of-line blocking]].