Difficulty: Hard · Puzzle 5 of 6 · Topic: Scoreboard Puzzles: Mismatch or Ordering Bug?
Random tests fail with 'expected queue not empty' in about 2% seeds. DUT has variable response latency and can legally return after stimulus stops.
task run_phase(uvm_phase phase);
forever begin
compare_once();
end
endtask
function void check_phase(uvm_phase phase);
if (exp_q.size() != 0 || got_q.size() != 0)
\`uvm_error("SB", "leftover transactions");
endfunctionA scoreboard needs explicit drain policy when response latency is elastic.
1) Log queue sizes near end-of-test and observe they are shrinking but not yet zero.
2) Confirm no drain window exists between stimulus completion and final check.
3) Implement phase_ready_to_end wait/drain with bounded timeout and progress checks.
4) Keep objection until queues settle or timeout indicates real hang.
5) Track residual ids in timeout message for actionable debug.Answer: Bug: scoreboard checks leftovers immediately without allowing legal late responses to drain. Fix: add bounded end-of-test draining before final emptiness assertion.
Removing the check hides escapes; the right fix is controlled drain, not silence.
What timeout heuristic would you use so drain does not mask real deadlocks?