Interview Question · Design
Doubling Throughput: A Micro-Architecture Call
The Question
The interviewer asks
Your packet-processing block meets timing at 1 GHz but must now handle twice the traffic. Walk through your options and how you would choose.
You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
Start with questions: is the limit the clock, the datapath width, or dependencies between packets? Then compare three options. A deeper pipeline adds latency and hazards. Two lanes double area and need ordering at the merge. A wider datapath processes two units per cycle, but needs alignment logic. Choose by area, power, latency budget and verification effort, and prototype the risky part first.
- Deeper
A shorter stage allows a faster clock. Each new stage adds a cycle of latency and a hazard to check.
- Lanes
Two copies work on alternate packets. The merge must put the results back in order.
- Wider
One copy handles two units per cycle. Packets that do not fill a word need alignment logic.
Read the diagram as text
- Packets in: the stream to process
- Deeper pipeline: more stages: latency and hazards
- Two lanes: two copies: double area
- Wider datapath: two units per cycle: alignment logic
- Merge in order: tags restore packet order
- Packets out: same order as they came in
- Packets in → Deeper pipeline
- Packets in → Two lanes: split
- Packets in → Wider datapath
- Two lanes → Merge in order
- Deeper pipeline → Packets out
- Merge in order → Packets out
- Wider datapath → Packets out
Each option costs something different: latency, area or alignment logic. The table below compares them.
| Option | Gains | Costs | Main risk |
|---|---|---|---|
| Deeper pipeline, faster clock | Small area | Latency, more flops, clock power | Hazards and stalls between dependent packets |
| Two parallel lanes | Keeps the clock and per-packet logic | About twice the area | Reordering at the merge |
| Twice the datapath width | Same clock, one control path | Wider muxes and alignment | Packets that end mid-word |
A senior answer chooses with numbers: area and power budgets, the latency limit, and what the verification plan must add.
Self-Check
Tick each key point you covered in your answer.
What Interviewers Listen For (Senior)
A good senior answer asks what limits throughput today before choosing. It names one cost and one verification risk for each option.
What Interviewers Listen For (Staff)
A good staff answer ties the choice to the product: the power budget, the latency other blocks accept, and schedule risk. It plans an early prototype to measure the riskiest number.
Cross-Questions
Where the interviewer goes next. Open a node to practise that follow-up.
What IfProve ItCompareScale It
- Doubling Throughput: A Micro-Architecture Call
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CompareA Wider Datapath Versus Two LanesL3 Senior
Compare doubling the datapath width with building two independent lanes.
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
A wider datapath keeps one control path and one ordering, but packets rarely fill whole words. So it needs alignment and partial-word handling. Two lanes keep the narrow logic, but each lane needs control and the merge must restore order. Short packets favour lanes; long streaming data favours width.
Self-Check
Tick each key point you covered in your answer.
Missed Corner
"Width is free if the bus is already wider."
The processing logic must handle two units at once, including a packet ending in the first half. That rare case is where bugs hide.
Source Lesson R8.3 review claims (AXI-Stream filter: TKEEP and packet ends)
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Scale ItThe Verification Cost of an Architecture ChoiceL3 SeniorL4 Staff
How does each option change the verification plan, and how should that affect the choice?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
A deeper pipeline adds hazard and stall scenarios, which need directed tests and coverage of back-to-back dependencies. Two lanes add ordering checks at the merge and tests that make the lanes finish out of order. A wider datapath adds alignment corners. Count that work in weeks and put it next to the area and power numbers.
Self-Check
Tick each key point you covered in your answer.
Wrong Altitude
"Verification will adapt to whatever we build."
The verification effort is often the schedule. An option that saves 5% area but adds a reorder buffer can cost more time than it saves.
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What IfWhen Packets Depend on Each OtherL3 Senior
Asked if they choose the deeper pipeline
What if each packet updates a counter table that the next packet may read?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Then a deeper pipeline creates a read-after-write hazard. The next packet may read the table before the earlier update has been written back. You need forwarding of the in-flight update, or a stall when the same entry is used. Both add logic and rare cases that the verification plan must cover.
Self-Check
Tick each key point you covered in your answer.
Half-Truth
"Pipelining never changes the function."
It does not change the function of a pure datapath. With shared state, a read can overtake a write that is still in the pipe.
Source Lesson R4.6 review claims (registers cannot be moved into a feedback loop without changing the result)
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Prove ItSolve R5.6.1: A Merger That Favours A Two to OneSimulator-graded
Show it, don't just say it: this Silicon Lab problem is graded by the simulator.
What People Get Wrong
Common wrong answers and why they fail.
Wrong Altitude
Answers "pipeline it deeper" at once.
A deeper pipeline only helps if the clock can rise and packets are independent. Without asking, the candidate may solve the wrong limit.
Delivery
Lists the three options with no way to choose between them.
The interviewer wants the decision rule: which numbers decide it and who owns them. A list without a decision is a junior answer.
Missed Corner
"Two lanes are simple: duplicate the block."
Packets from one flow may need to leave in order. The merge then needs sequence tags or a reorder buffer, which is new logic to verify.
Source Lesson P1.7 review claims (ordering, tags and out-of-order completion)