VLSI DV Interview Puzzles · All levels

try_get Busy Spin at Time 0

No deadlock occurs, but simulation appears frozen at t=0. Explain root cause and proper fix.

Puzzle

Difficulty: Easy · Puzzle 3 of 6 · Topic: Semaphore & Mailbox Puzzles

No deadlock occurs, but simulation appears frozen at t=0. Explain root cause and proper fix.

Code

systemverilog

module tb;
  mailbox #(int) mb = new();
  int item;
  initial begin
    forever begin
      if (!mb.try_get(item)) begin
        // no delay, no event wait
      end
    end
  end
endmodule

Hint

A non-blocking API still needs timing control in polling loops.

Step-by-step solution

diagram

1) try_get returns immediately when mailbox empty.
2) Loop has no blocking statement or delay.
3) Process consumes simulator cycles in same time slot, often stuck at t=0.
4) Add blocking get, wait event, or small delay/backoff in empty case.

Answer

Answer: This is a zero-delay busy spin, not a protocol deadlock. Add wait/delay or switch to blocking get.

Why candidates get it wrong

Non-blocking call does not imply low-overhead loop by itself.

Interviewer follow-up

What backoff policy would you use to avoid both starvation and performance collapse?

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