RISC-V Design · All levels

Performance Tuning and Signoff: Worked Example

Worked Example for Performance Tuning and Signoff.

Worked example

Worked Example for Performance Tuning and Signoff is anchored on P95 and P99 workload latency, sustained throughput per subsystem, and power-performance target closure against signoff criteria.. Convert observations into mechanism-backed decisions with explicit ownership.

A regression flags P95 and P99 workload latency, sustained throughput per subsystem, and power-performance target closure against signoff criteria.. Strong closure isolates first failing mechanism, proves causality, applies one bounded change, and validates blast radius.

System view

diagram

RISC-V PIPELINE DIAGRAM - Performance Tuning and Signoff

PC -> IF -> ID -> EX -> MEM -> WB
      |     |      |      |      |
  i-cache decode  ALU/BR  LSU    regfile write
              \   |
               +-> branch resolve + redirect

Hot paths:
  - branch + load-use dependencies in ID/EX
  - memory latency stretching MEM stage
  - writeback arbitration for integer/vector units

Focus: keep control hazards predictable

Evidence matrix

diagram

RISC-V EVIDENCE MATRIX - Performance Tuning and Signoff

+--------------------------+--------------------------------+--------------------------------+---------------------------+
| Evidence                 | Tells you                      | Does not prove                 | Next action               |
+--------------------------+--------------------------------+--------------------------------+---------------------------+
| perf counter timeline    | where regression appears       | exact mechanism causality      | correlate with trace      |
| decode/control dump      | control intent per instruction | pipeline side-effect ordering  | inspect retire semantics  |
| trap + CSR logs          | privilege/fault behavior       | performance bottleneck alone   | pair with CPI buckets     |
| MMU/TLB walk trace       | translation behavior           | full system QoS impact         | test mixed workloads      |
| post-fix trend graph     | movement after fix             | long-term stability            | run stress matrix         |
+--------------------------+--------------------------------+--------------------------------+---------------------------+

Capture baseline and failing traces under fixed metadata tags.
Classify stage loss and dominant mechanism.
Collect Performance signoff pack: workload matrix, counter baseline snapshots, tuning decision log, and acceptance report with guardrail compliance evidence..
Apply one bounded fix with owner signoff.
Run validation matrix and decide ship/rollback.

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