66 lines
2.7 KiB
Markdown
66 lines
2.7 KiB
Markdown
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# Phase 70: Refill Tuning Prerequisites (Observability SSOT)
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**Status**: 🟡 ACTIVE
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**Context**:
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- Current baseline (Mixed WS=400 + prefault) generates almost **zero cache misses** (refills) in Unified Cache.
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- Optimizing `unified_cache_refill()` or Warm Pool logic will yield **zero throughput gain** if this path is not hot.
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- Phase 70 must be gated on *observing significant refill activity*.
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## 1. Observability Protocol (Step 0)
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Before implementing any refill/WarmPool changes, execute this sequence:
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1. **Build with Stats**:
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```bash
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make bench_random_mixed_hakmem_observe EXTRA_CFLAGS='-DHAKMEM_UNIFIED_CACHE_STATS_COMPILED=1'
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```
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*(Note: Phase 70-0 fixed release-mode stats blocking)*
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2. **Run with Stats**:
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```bash
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HAKMEM_WARM_POOL_STATS=1 ./bench_random_mixed_hakmem_observe 20000000 400 1
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```
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3. **Check Output**:
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- Look for `Unified-STATS`: `miss=...`
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- Look for `WarmPool-STATS`: `hits=...`
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**Decision Gate**:
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- If `miss` counts are < 1000 (approx <0.01% miss rate):
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- **STOP**: Optimization has no ROI on this workload.
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- **ACTION**: Either accept current state (refill is not a bottleneck) or switch to a research workload (below).
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- If `miss` counts are significant:
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- **GO**: Proceed with Phase 70 logic changes.
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## 2. Research Workload (If Refill Optimization is Mandatory)
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If you must measure refill performance improvements (e.g., for architectural validation), modify the workload to force cache pressure:
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**Option A: Disable Prefault (Cold Path Stress)**
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```bash
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HAKMEM_BENCH_PREFAULT=0 ./bench_random_mixed_hakmem_observe ...
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```
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- Pros: Forces refills during benchmark loop.
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- Cons: Measures startup behavior, not steady-state throughput.
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**Option B: Increase Working Set (Steady State Stress)**
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```bash
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./bench_random_mixed_hakmem_observe 20000000 8192 1 # WS=8192 > Cache(2048)
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```
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- Pros: Forces steady-state evictions/refills.
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- Cons: Different workload profile than standard Mixed SSOT (WS=400).
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**WARNING**: Do NOT change `HAKMEM_TINY_STATIC_ROUTE` or `ULTRA` flags unless specifically testing routing changes. The default `LEGACY` route *does* use Unified Cache for alloc/free.
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## 3. Reference: Why "LEGACY" Route is OK
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- **LEGACY** route means "not ULTRA/MID/V7 specialized".
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- Alloc path: `malloc_tiny_fast` → `tiny_hot_alloc_fast` → **Unified Cache (TLS array)**.
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- Free path: `free_tiny_fast` → `tiny_hot_free_fast` → **Unified Cache (TLS array)**.
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- Refill path: Cache Miss → `unified_cache_refill` → **Warm Pool** → Registry.
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Previous confusion ("LEGACY unused") was due to:
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1. Stats counting was gated by `#if !HAKMEM_BUILD_RELEASE`.
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2. Low miss rate in WS=400 made it look unused.
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3. Phase 70-0 fixed the stats visibility.
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