Closing the Last Eight Percent

Where the interviewer goes next. Open a node to practise that follow-up.

WhyWhat IfCompareScale It

1. Closing the Last Eight Percent
2. WhyWhy the Last Few Percent Take the LongestL1 JuniorL2 Mid-Level

   Why does coverage rise fast at first and then crawl?

   Reveal the Model Answer

   Early samples land on empty bins, so almost every sample adds coverage. Later, most samples land on bins already hit, and only the rare ones add anything. Bins with low probability under the constraints may take thousands of samples each. Holes that cannot be reached never close, so they look like a crawl that never ends.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Early samples land on empty bins, so almost every sample adds coverage. Key point 2 Later, most samples land on bins already hit, and only the rare ones add anything. Key point 3 Bins with low probability under the constraints may take thousands of samples each. Key point 4 Holes that cannot be reached never close, so they look like a crawl that never ends.

   Half-Truth

   "Slow progress means the tests are bad."

   Some slowdown is normal arithmetic: the last empty bins are the hardest to hit by chance. A plateau, where progress stops, is the real warning.

   Source Spear & Tumbush, SystemVerilog for Verification (3rd ed.), §9.1 (PDF p361)

   • Lesson C7Closing Holes
3. Scale ItReporting Closure ProgressL3 Senior

   Your lead wants a weekly closure report for a subsystem. What goes in it besides one percentage?

   Reveal the Model Answer

   Show coverage per plan feature, not one total, with its trend over the weeks. Show how many new bins each regression adds, because a flat line means you need new work, not more runs. Add the open holes grouped by cause and owner, and the bug rate. A falling bug rate with flat coverage is the pattern you want before sign-off.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Show coverage per plan feature, not one total, with its trend over the weeks. Key point 2 Show how many new bins each regression adds, because a flat line means you need new work, not more runs. Key point 3 Add the open holes grouped by cause and owner, and the bug rate. Key point 4 A falling bug rate with flat coverage is the pattern you want before sign-off.

   Wrong Altitude

   "One merged percentage is enough for management."

   One number hides which feature is behind and why. A 95% total can mean one critical feature at 40%.

   Source Mehta, SystemVerilog Assertions and Functional Coverage (3rd ed.), §25.5 (PDF p446)

   • Lesson C7Closing Holes
   • Lesson U15.1Coverage Architecture and Planning
4. CompareBiasing Random Stimulus vs a Directed TestL2 Mid-Level

   Compare biasing the constraints toward a hole with writing a directed test for it.

   Reveal the Model Answer

   Biasing, for example with dist weights or a test-level constraint, keeps the other fields random. So the rare case also meets many neighbours. A directed test hits the case once, fast and repeatably. Bias first when the case is reachable but rare. Go directed when it needs a long exact sequence, such as an error during a full FIFO after reset.

   Read the diagram as text

   • initial block: round 1, then round 2
   • p.randomize(): class constraints only
   • p.randomize() with: the kind and class of one hole
   • record(p): cov.sample(p); hits[k][c]++
   • cov: pkt_cross: 7 legal cells
   • hits[kind][class]: the same cells, counted

   • initial block → p.randomize(): randomize
   • initial block → p.randomize() with: aimed randomize
   • initial block → record(p): record(p)
   • record(p) → cov: sample(p)
   • record(p) → hits[kind][class]: count
   • hits[kind][class] → initial block: holes()

   The hole list comes from the counts, so each aimed packet goes exactly where coverage is missing, and nowhere else.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Biasing, for example with dist weights or a test-level constraint, keeps the other fields random. Key point 2 So the rare case also meets many neighbours. Key point 3 A directed test hits the case once, fast and repeatably. Key point 4 Bias first when the case is reachable but rare. Key point 5 Go directed when it needs a long exact sequence, such as an error during a full FIFO after reset.

   Wrong Altitude

   "Directed tests are old-fashioned; always bias."

   Some holes need a precise sequence of events that random stimulus almost never builds. A directed test is the cheapest way there.

   Source Spear & Tumbush, SystemVerilog for Verification (3rd ed.), §9.1 (PDF p362)

   • Lesson 4.2Shallow Copies and Deep Copies
   • Lesson C7Closing Holes
5. What IfTape-Out with Coverage Holes LeftL3 SeniorL4 Staff

   What if tape-out is in a week and 3% is still open? What do you do?

   Reveal the Model Answer

   Go through each remaining hole with the designers and the project lead. For each, state what feature it covers, how likely the case is in real use, and what a bug there would cost. Close the high-risk ones with targeted tests or formal. For the rest, record an accepted risk with an owner, plus a plan for post-silicon tests or a software workaround.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Go through each remaining hole with the designers and the project lead. Key point 2 For each, state what feature it covers, how likely the case is in real use, and what a bug there would cost. Key point 3 Close the high-risk ones with targeted tests or formal. Key point 4 For the rest, record an accepted risk with an owner, plus a plan for post-silicon tests or a software workaround.

   What Interviewers Listen For (Staff)

   A good staff answer turns the hole list into a risk register that the lead signs. Each accepted item gets a post-silicon test plan.

   Delivery

   "Waive everything open; the date is fixed."

   A blanket waiver hides the one hole that matters. Each item needs a decision, even if most of them are accepted.

   • Lesson C7Closing Holes