54 lines
3.2 KiB
Markdown
54 lines
3.2 KiB
Markdown
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CPU Hotpath Overview (bench profile)
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====================================
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Context
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-------
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- Build/profile: `HAKMEM_PROFILE=bench`, `HAKMEM_TINY_PROFILE=full`, `HAKMEM_WARM_TLS_BIND_C7=2`.
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- Workloads sampled:
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- 16–1024B (`./bench_random_mixed_hakmem 1000000 256 42`)
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- 129–1024B (`HAKMEM_BENCH_MIN_SIZE=129 HAKMEM_BENCH_MAX_SIZE=1024 ./bench_random_mixed_hakmem 1000000 256 42`)
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- Target: identify user‑space hot spots to guide C7 flattening work.
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Sampling attempt (perf)
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-----------------------
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- `perf record -g -e cycles:u` and `perf record -g -e cpu-clock:u` both fall back to `page-faults` on this host (likely perf_event_paranoid). The captures show:
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- ~97% of page-fault samples in `__memset_avx2_unaligned_erms` during warmup/zeroing.
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- Callers were `tiny_tls_sll_drain.part.0.constprop.0` and `adaptive_sizing_init` (warmup path).
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- No steady‑state cycle profile was available without elevated perf permissions. `perf data` removed after inspection (`rm perf.data`) to keep tree clean.
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What we can infer despite the limitation
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----------------------------------------
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- Warmup zeroing dominates page‑fault samples; steady‑state alloc/free is not represented.
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- Hot candidates for the next pass (from previous code inspection and bench intuition):
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- `tiny_alloc_fast` / `malloc_tiny_fast` (C7 fast path)
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- `hak_tiny_free_fast_v2`
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- `tiny_unified_cache` hit path helpers
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- `tls_sll_pop_impl` / `tiny_tls_sll_drain`
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Next measurement options
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------------------------
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- If perf cycles are still blocked:
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- Use `perf stat -e cycles,instructions,branches,branch-misses -r 5 -- ...` to get aggregate IPC per workload.
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- Add temporary user‑space counters (Box‑guarded) around C7 alloc/free hot sections to estimate per‑op cycles.
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- Run perf with elevated permissions or lower `perf_event_paranoid` if available.
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Aggregate perf stat snapshot (bench profile)
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--------------------------------------------
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- Env: `HAKMEM_PROFILE=bench HAKMEM_TINY_PROFILE=full HAKMEM_WARM_TLS_BIND_C7=2`
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- Workloads (Release, 3× perf stat):
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- 16–1024B: cycles≈109.8M, instructions≈233.2M → IPC≈2.12, branches≈49.4M, branch-miss≈2.89%
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- 129–1024B: cycles≈109.6M, instructions≈230.3M → IPC≈2.10, branches≈48.8M, branch-miss≈2.90%
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- 16–1024B with `HAKMEM_TINY_C7_HOT=1` (UC hit-only + flat TLS→UC→cold):
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- cycles≈111.8M, instructions≈242.1M → IPC≈2.16, branches≈52.0M, branch-miss≈2.75%
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- RSS≈7.1MB; throughput ≈47.4–47.6M ops/s (hot=1) vs ≈47.2M (hot=0) on the same runset.
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Action items flowing from this note
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-----------------------------------
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- Proceed with design notes for C7 alloc/free flattening and UC hit simplification based on code structure.
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- Keep warmup zeroing out of the steady‑state loop when profiling (consider `HAKMEM_BENCH_FAST_MODE` for future captures).
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Conclusion (current state)
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--------------------------
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- `HAKMEM_TINY_C7_HOT` は実験用フラグとして残し、デフォルト OFF のまま運用する。ON にしても branch-miss はわずかに改善する程度で、ops/s は同等〜微減。
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- ひとまず「安全＋そこそこ速い」現行経路を基準とし、さらなるフラット化は別途必要性を見て検討する。***
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