U. Glaeser

a common register space, it is important that we still provide a separate register file in each PE to support
fast register access, as it is difficult for a centralized register file to provide a 1-cycle multi-port access
time with today’s high clock rates. This decentralization can be achieved in two ways, both of which
provide faster register access times due to physical proximity and fewer access ports per physical register
file.
• RF Partitioning: In this approach, each physical register file implements (or maps) an independent
set of ISA-visible registers. Notice that a PE may occasionally need a register value stored in a
nonlocal register file, in which case the value is fetched through an interconnection network that
interconnects the PEs.
• RF Replication: With the replication scheme, a physical copy of the register file is kept in each PE,
so that each PE has a local copy of the shared set register space. These register file replica maintain
different versions of the register space, i.e., the multiple copies of the register file store register
values that correspond to the processor state at different points in a sequential execution of the
program. In general, replication avoids unnecessary communication; however, if not done carefully,
it might increase communication by replicating data that is not used in the future. A
multithreaded processor that uses the replication scheme is the multiscalar processor [9].
PE Interconnect for Register Values
When threads share a common register space, and a distributed RF structure is used, an important
hardware attribute is the type of interconnect used to send register values from one PE to another. The
interconnects that have been proposed in the context of multithreaded processors are bus, ring (unidirectional
and bi-directional), crossbar, mesh, and hypercube; of course, it is possible to use other types
of interconnects as well.
Bus: The bus is a simple, fully connected network. However, it permits only one data transmission at
any time, providing a bandwidth of only O(1). In fact, the bandwidth scaling is worse than O(1) because
of reduction in bus operating speed with the number of ports, due to increase in capacitance. Therefore,
it may be a poor choice as an interconnect for inter-PE register communication, which may be nontrivial,
especially when using a large number of PEs.
Crossbar: A crossbar interconnect also provides full connectivity from every PE to every other PE. It
provides O( N)
bandwidth, but the cost of the interconnect is proportional to the number of cross-points,
2
or O( N ). When using a crossbar, all PEs are of same proximity to each other; hence the thread allocation
algorithm becomes straightforward; however, a crossbar may not scale as easily as a ring or mesh. It is
important to note that fast crossbars can be built on a single chip. With a crossbar-type interconnect,
there is no notion of neighboring PEs, so all PEs become equally far away. Therefore, the cross-chip wire
delays begin to dominate the inter-PE communication latency.
Ring: With a ring-type interconnect, the PEs are connected as a circular loop, and there is a notion of
neighboring PEs and distant PEs. Routing in a ring is trivial because there is exactly one route between
any pair of nodes (two routes if it is a bi-directional ring). The ring can be easily laid out with O( N)
space using short wires (as depicted in Fig. 5.15), which can be easily widened. A ring is ideal if most of
the inter-PE register communication can be localized to neighboring PEs (which is typically the case in
a sequential threads processor that uses the circular queue PE organization [36]), but is a poor choice if
a lot of communication happens across distant PEs. An advantage of the ring is that it easily supports
the scaling up of the number of PEs, as allowed by technological advances.
Mesh: Rings generalize naturally to higher dimensions, including 2D grids and 3D cubes (with endaround
connections). The main advantages of mesh are its regular structure and its ability to provide
full connectivity between four neighboring PEs (as opposed to two PEs with the ring). Similar to a ring, a
mesh can easily support the scaling up of the number of PEs. The mesh suffers from the same disadvantages
of a ring in communicating with distant PEs. Moreover, thread allocation for a mesh topology is
more complex than that for ring and crossbar.
© 2002 by CRC Press LLC