Suppose we have 6 pipeline stages naming S1, S2, S3, S4, S5, S6 with stage…
2024
Suppose we have 6 pipeline stages naming S1, S2, S3, S4, S5, S6 with stage latencies as 110, 340, 400, 380, 420, 410 nanoseconds respectively. Every pipeline stage have a buffer latency of 10 nanoseconds buffer latency. If one of the pipeline stages is split into 2 equal halves to increase the throughput. Which one of the following stages has to be split to increase the throughput?
- A.
S1
- B.
S3
- C.
S5
- D.
S6
Attempted by 30 students.
Show answer & explanation
Correct answer: C
Key idea: The pipeline cycle time is determined by the maximum total delay among stages, where total delay = stage latency + buffer latency.
S1 total = 110 + 10 = 120 ns
S2 total = 340 + 10 = 350 ns
S3 total = 400 + 10 = 410 ns
S4 total = 380 + 10 = 390 ns
S5 total = 420 + 10 = 430 ns
S6 total = 410 + 10 = 420 ns
Identify the bottleneck: the maximum total delay is 430 ns at S5, so the current cycle time is 430 ns.
If S5 is split into two equal halves: each half has latency 420 / 2 = 210 ns. Adding the buffer gives 210 + 10 = 220 ns per new stage.
After splitting S5, the other stages remain the same, so the largest total delay becomes S3 at 410 ns. The new cycle time is 410 ns, down from 430 ns, so throughput increases.
Conclusion: Split S5 because it has the largest total delay; splitting it reduces the pipeline's cycle time the most.