Computer Architecture · All levels

Routing and Flow Control — Step-by-Step Walkthrough

Step-by-Step Walkthrough for Routing and Flow Control (NoC and Interconnect Architecture).

Step-by-step analysis walkthrough

Follow this walkthrough when you own Routing and Flow Control in a performance review or architecture signoff meeting.

Confirm workload and analysis tag.
Open Routing/flow-control stress verification report and capture worst cluster.
Classify bottleneck type.
Map cluster to structure.
List competing hypotheses.
Run cheapest falsifying experiment.
Estimate metric delta.
Choose bounded change.
List regression surfaces.
Replay workloads.
Write decision memo.
Capture methodology guardrail.

Artifacts to collect

Workload list
PMU/trace config
Metric dashboard
Decision memo

Decision memo template

diagram

DECISION MEMO — Routing and Flow Control
metric:
hypothesis:
experiment:
decision:
validation:

Architecture deep dive

NoC is a queueing system — bandwidth, latency, and deadlock are coupled.

Concept diagram

diagram

NoC TOPOLOGY SKETCH

CPU0 ──┐      ┌── LLC0 ── DRAM0
       R0 ─── R1
CPU1 ──┘      │
              R2 ─── R3 ── GPU/DMA
              │      │
             NPU    LLC1 ── DRAM1

Look for: hot links, cyclic dependencies, VC starvation, and tail latency.

Metric graph

diagram

LATENCY DISTRIBUTION

p50    ██████  32 ns
p90    ████████████  71 ns
p99    ████████████████████████  210 ns
p99.9  █████████████████████████████████  480 ns

Averages hide QoS failures.

Metrics and artifacts

link utilization
average latency by master
retry/backpressure counts
QoS violation log

Mini case study

Average latency looks fine but tail latency spikes for CPU coherent reads when GPU DMA runs. QoS and separate VCs fix the starvation without doubling link width.

Debug branches

If deadlock, check credit loops and routing restrictions first.
If latency tail long, inspect arbitration and buffer depth.

Senior review question

Ask: what single metric would prove this concept is working or failing on your workload?

Key takeaways

Connect every architecture claim to a workload and measurable metric.
State verification and PPA impact before proposing design changes.

Common pitfalls

Feature-driven design without MPKI/IPC/bandwidth evidence.
Ignoring coherency and NoC traffic in cache and accelerator sizing.

Study notes

Re-read this topic with one concrete workload.

Practice this lesson