Accepted traffic
(flits/cycle/IP)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020
Offered load (packets/cycle/IP)
85
Evaluation of Network-on-Chip Architectures
4.4.1 Accepted Traffic versus Offered Load
The accepted traffic depends on the rate at which the IP blocks inject traffic
into the network. Ideally, accepted traffic should increase in direct proportion to the increasing offered load. However, due to the limitation of routing
and arbitration strategy and the unavailability of enough buffer space within
the WH router (FIFO depth being much lesser than the size of the packet),
the network suffers from contention. Therefore, the accepted traffic saturates after a certain value of the offered load. Figure 4.6 depicts this scenario
for uniformly distributed traffic in a MoT-based network. The maximum
accepted traffic where the network is saturated is termed as throughput and
it relates to the maximum sustainable data rate by the network.
4.4.2 Throughput versus Locality Factor
Locality factor is defined as the ratio of local traffic to total traffic and its
value is zero for uniformly distributed traffic. For a 4 × 4 MoT, as shown in
Figure 4.7, the possible distances (d) of the destinations from any source are
d = 0, 2, 4, 6, and 8. There is only one destination core at the nearest cluster
(d = 0). For example, if the locality factor is 0.5, 50% of traffic is targeted at the
nearest cluster from source. The rest of the traffic is distributed according to
their distances from the source, such that a destination at the nearer cluster
gets more traffic compared to one at farther cluster. In a 4 × 4 MoT, for a locality factor of 0.5, the distribution of the traffic is as follows:
• Fifty percent of the total traffic goes to the cluster having d = 0.
• Rest 50% of the traffic is distributed as follows:
• (8 * 50)/(2 + 4 + 6 + 8) = 20% of the total traffic goes to the cluster
having d = 2
Figure 4.6
Accepted traffic with varying offered load at uniformly distributed traffic.