8. CASE STUDIES
the packet right back along y-. When the y+ link is not present (at the edge of the mesh), the second
choice is y-. When the desired route is to travel in the z- direction, the logic must follow the z- path
to ensure there are no broken links at all on the path to the final destination. If one is found, the
route is forced to z+, effectively forcing the packet to go the long way around the Z torus.
8.2.3 FLOW CONTROL
Buffer resources are managed using credit-based flow control at the data-link level. The link control
block (LCB) is shown at the periphery of the Seastar router chip in Figure 8.6 . Packets flow across
the network links using virtual cut-through flow control — that is, a packet does not start to flow
until there is sufficient space in the receiving input buffer. Each virtual channel (VC) has dedicated
buffer space. A 3-bit field (Figure 8.7 ) in each flit is used to designate the virtual channel, with a
value of all 1's representing an idle flit. Idle flits are used to maintain byte and lane alignment across
the plesiochronous channel. They can also carry VC credit information back to the sender.
SEASTAR ROUTER MICROARCHITECTURE
(a) Seastar block diagram.
(b) Seastar die photo.
Figure 8.6: Block diagram of the Seastar system chip.
Network packets are comprised of one or more 68-bit flits (flow control units). The first flit of
the packet (Figure 8.7 )isthe header flit and contains all the necessary routing fields (destination[14:0],
age[10:0], vc[2:0]) as well as a tail (t) bit to mark the end of a packet. Since most XT networks are
on the order of several thousand nodes, the lookup table at each input port is not sized to cover the