58 lines
1.6 KiB
Markdown
58 lines
1.6 KiB
Markdown
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# Phase 80-1: Inline Slots Switch Dispatch — Results
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## Goal
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Reduce per-op comparison/branch overhead in inline-slots routing for the hot classes by replacing the sequential `if (class_idx==X)` chain with a `switch (class_idx)` dispatch when enabled.
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Scope:
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- Alloc hot path: `core/box/tiny_front_hot_box.h`
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- Free legacy fallback: `core/box/tiny_legacy_fallback_box.h`
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## Change Summary
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- New env gate box: `core/box/tiny_inline_slots_switch_dispatch_box.h`
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- ENV: `HAKMEM_TINY_INLINE_SLOTS_SWITCHDISPATCH=0/1` (default 0)
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- When enabled, uses switch dispatch for C4/C5/C6 (and excludes C2/C3 work, which is NO-GO).
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- Reversible: set `HAKMEM_TINY_INLINE_SLOTS_SWITCHDISPATCH=0` to restore the original if-chain.
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## A/B (Mixed SSOT, 10-run)
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Workload:
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- `ITERS=20000000`, `WS=400`, `RUNS=10`
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- `scripts/run_mixed_10_cleanenv.sh`
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Results:
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Baseline (SWITCHDISPATCH=0, if-chain):
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- Mean: `51.98M ops/s`
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Treatment (SWITCHDISPATCH=1, switch):
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- Mean: `52.84M ops/s`
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Delta:
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- `+1.65%` ✅ **GO** (threshold +1.0%)
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## perf stat (single-run sanity)
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Key deltas (treatment vs baseline):
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- Cycles: `-1.6%`
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- Instructions: `-1.5%`
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- Branches: `-2.9%` ✅
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- Cache-misses: `-6.7%`
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- Throughput (single): `+3.7%`
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Interpretation:
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- Switch dispatch removes repeated failed comparisons for the hot inline-slot classes, reducing branches/instructions without causing cache-miss explosions.
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## Promotion
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Promoted to Mixed SSOT defaults:
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- `core/bench_profile.h`: `HAKMEM_TINY_INLINE_SLOTS_SWITCHDISPATCH=1`
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- `scripts/run_mixed_10_cleanenv.sh`: `HAKMEM_TINY_INLINE_SLOTS_SWITCHDISPATCH=1`
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Rollback:
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```sh
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export HAKMEM_TINY_INLINE_SLOTS_SWITCHDISPATCH=0
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```
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