A fix is a prediction until counters confirm it. Re-run the same workload and compare the same events, not summary IPC alone.
The cache-storm fix predicted MPKI 9. Stalls are 9 x 40 / 1000 = 0.36 CPI, so CPI is 1.86 and IPC is 0.54, within about 2 percent of the 0.55 baseline.
Fixed: CPI = 1.5 + 9 x 40 / 1000 = 1.86, IPC = 1 / 1.86 = 0.54
Base: IPC 0.55, gap 1 - 0.54 / 0.55 = 0.02, about 2 percent
Amdahl: kernel covers 0.7 of time, 4x faster: 1 / (0.3 + 0.7 / 4) = 2.1xThe stall model undercounts another class: re-split stalls by level, TLB and bandwidth. One fixed metric does not close the case.
Every fix spends area, power or complexity. State the price beside the gain, with an owner for each.
The padding fix spends SRAM bytes to save fabric power. The class split spends buffers to remove deadlock. Say both aloud.
Fix in this topic | Gain | Price and owner |
|---|---|---|
Page mapping restore | 7 MPKI back | OS policy owner, no silicon cost |
Request-response classes | Deadlock removed | Buffer area, NoC owner |
DMA rate cap | 30 GB/s under capacity | Burst latency, QoS owner |
Weight tiling | 0.775 ms under target | SRAM tile, accelerator owner |
Counter padding | 41 GB/s removed | Cache bytes, software owner |
Interviewers reuse a small trap set across scenarios. Naming each trap early shows seniority more than any metric.
The table below collects every trap from the four scenarios. Each reply redirects to a counter or an experiment.
Trap | Calm reply |
|---|---|
Bigger cache | Size the conflict fraction first |
More links or channels | Split classes first, then size |
Peak TOPs | Publish sustained latency at stated intensity |
Faster fabric | Remove the traffic before moving it |
Average IPC only | Split core IPC from system bytes |
Close in thirty seconds: workload, root cause, number, fix, validation. One sentence each, then stop.
Example: burst workload, page-policy conflicts, 15 percent IPC loss, mapping restored, MPKI 9 on traces. Short endings pass.
Baseline vs fixed counters on the same workload, the arithmetic linking them, and the PPA price with an owner. Method on one slide.