Bandwidth & Latency Budgeting: Interview Drills

PROMPT
You see sustained bandwidth, p99 latency, utilization, head-of-line blocking on Bandwidth & Latency Budgeting. Walk through root cause and fix.

STRONG ANSWER
1. Names the layer and transaction identity.
2. Explains burst length, outstanding depth, arbitration, and packet overhead convert interface width into real workload throughput.
3. Requests bandwidth budget sheet, latency histogram, traffic replay summary.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to widening the interface, increasing FIFO depth, or blaming firmware without evidence.

LATENCY vs OFFERED LOAD

latency
  ^                                   *
  |                                 *
  |                              *
  |                          *  <- knee: queues build fast
  |                  *  *
  |   *  *  *  *
  +--------------------------------------> offered load (% of peak)
   0%        50%        80%   90%  100%

Lesson: usable bandwidth ends at the knee, not at 100% peak.

ROOT-CAUSE TREE — Bandwidth & Latency Budgeting

sustained bandwidth, p99 latency, utilization, head-of-line blocking looks wrong
        |
   reproducible?
     /        \
   no          yes
   |            |
 flaky env   same first transaction every time?
 / seed         /            \
              yes             no
               |               |
        protocol rule     timing/reset/PVT
        or config bug     or load-dependent

FUNDAMENTALS STACK

software intent
     |
transaction (ID, addr, len, attr, order)
     |
link/channel (handshake, credit, retry)
     |
physical (clock, reset, lanes, PHY)

Debug golden rule: never change layers without carrying transaction identity.

STALL BREAKDOWN EXAMPLE

ready stalls      ████████████████  42%
credit wait       ██████████        26%
ordering block    ██████            16%
reset/config      ████              10%
other             ██                6%

If ready stalls dominate, widening the bus will not help.