82 lines
2.9 KiB
Markdown
82 lines
2.9 KiB
Markdown
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# SACS‑3 Overview (Phase 6.16)
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Goal: keep the hot path predictable and fast by deciding tiers by size only, and let ACE optimize just the cache layer (L1) dynamically.
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## Tiers
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- L0 Tiny (≤ 1 KiB)
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- TLS magazine, TLS Active Slab, MPSC remote‑free.
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- L1 ACE (1 KiB < size < 2 MiB)
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- MidPool: 2/4/8/16/32 KiB (5 classes)
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- LargePool: 64/128/256/512 KiB/1 MiB (5 classes)
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- W_MAX rounding: class `c` accepted if `c ≤ W_MAX × size`.
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- Gap 32–64 KiB absorbed by rounding to 64 KiB when within W_MAX.
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- L2 Big (≥ 2 MiB)
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- BigCache + mmap, THP gate.
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## Hot Path (hak_alloc_at)
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```
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if (size ≤ 1 KiB) return tinyslab_alloc(size);
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if (size < 2 MiB) return hkm_ace_alloc(size, site, P); // L1
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else return bigcache/mmap; // L2
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```
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Where `P` is a `FrozenPolicy` snapshot (RCU‑published); hot path reads it once per call.
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## ACE = “Smart Cache” for L1
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What ACE does (off hot path):
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- CAP: per‑class budget (front‑loading) for Mid/Large.
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- Site/class→shard: fix locality and reduce contention.
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- Free policy per class: KEEP / delayed MADV_FREE / batched DONTNEED.
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- W_MAX candidates: choose (e.g., {1.25, 1.5, 1.75}) via CANARY.
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- BATCH_RANGE: pick from a few candidates.
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- All decisions baked into a `FrozenPolicy`, published once per window.
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## Profiling & Overhead Tracking
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Enable sampling profiler:
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```
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export HAKMEM_PROF=1
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export HAKMEM_PROF_SAMPLE=10 # sample 1/1024
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```
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Key categories: `tiny_alloc`, `ace_alloc`, `malloc_alloc`, `mmap_alloc`, plus internals like `pool_lock/refill`, `l25_lock/refill`, and tiny internals.
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Sweep helper:
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```
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scripts/prof_sweep.sh -d 2 -t 1,4 -s 8
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```
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## Roadmap
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1. CAP tuning (static → learning):
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- Observe L1 `malloc_alloc` avg ns and L1 fallback rate per range.
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- Increase CAP where hit < target; decrease where overshoot.
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2. W_MAX tuning per tier (Mid vs Large) with guard rails.
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3. Shard routing via FrozenPolicy (reduce lock contention).
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4. Publish policy via `hkm_policy_publish()` on window boundaries (RCU).
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## Learning Axes & Controls
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- Axes:
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- Threshold (mmap/L1↔L2), Class Count (containers), Class Shape (boundaries + W_MAX), Class Volume (CAP)
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- Implemented now:
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- Soft CAP gating on Mid/L2.5 refills (CAP over → bundle=1, under → up to 4)
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- Learner thread adjusts `mid_cap[]/large_cap[]` by target hit rate (hysteresis, budget)
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- Env controls:
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- `HAKMEM_LEARN=1`, `HAKMEM_LEARN_WINDOW_MS`, `HAKMEM_TARGET_HIT_MID/LARGE`, `HAKMEM_CAP_STEP_MID/LARGE`, `HAKMEM_BUDGET_MID/LARGE`
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- `HAKMEM_CAP_MID/LARGE` (manual override), `HAKMEM_WMAX_MID/LARGE`
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- Planned: `HAKMEM_WRAP_L2/L25`, `HAKMEM_MID_DYN1`
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## Inline Policy (Hot Path)
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- Size-only tier selection; no syscalls on hot path.
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- Static inline + LUT for O(1) class mapping.
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- One `FrozenPolicy` load per call; otherwise read-only.
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- Site Rules stays off hot path (layer-internal hints only in future).
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