docs: per-switch N=1 local profile — shims are ~1% at N=1; cost is schedule_loop + run-queue + a hot-path futex_wake (revises handoff shim hypothesis to a many-core one)

2026-06-14 18:58:26 +00:00
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 # Per-switch cost — N=1 local profile (spike findings)
 Measured with `benches/switch_cost.rs` (local mode: one actor, one scheduler,
 tight `yield_now()` loop = one park/unpark round-trip with no IO/channel/timer
 and no cross-core traffic). Sandbox: 1 core, kernel 6.18, **no PMU** (hardware
 counters unavailable), so attribution is from `perf record -e task-clock`
 (software timer sampling) plus the bench's own rdtsc/wall brackets.
 ## Numbers (stable across runs)
 - Round-trip p50 ≈ **303 ns ≈ 828 cyc** (instrumentation floor subtracted).
 - Derived effective clock ≈ **2.73 GHz** (rdtsc cyc / wall ns — the two lenses
  corroborate, so the cycle counts are trustworthy).
 - p90 316 ns, p99 472 ns; max is a multi-ms OS-deschedule outlier (1 shared
  core) — ignore the max, trust the percentiles (the harness pools them).
 ## Attribution (perf task-clock, self-time, 28.6k samples / 12M round-trips)
 | share | symbol | bucket |
 |------:|--------|--------|
 | 24.8% | `runtime::schedule_loop` | scheduler logic (slot-word/epoch + dispatch) |
 |  8.6% | `MutexQueue::push`/`pop`/`len` | run-queue ops |
 | ~12%  | `do_syscall_64`+`syscall`+`futex_*` | **futex_wake on the hot path** |
 |  3.5% | `IoThread::drain_completions` | the always-on IO thread (`run()` starts one) |
 | ~30%  | `main` + `clock_gettime`/Timespec + `quicksort` | **instrumentation** (timing + percentile sort) |
 |  ~1%  | `switch_to_scheduler`+`switch_to_actor_asm`+ sp accessors | **the context shims + TLS** |
 ## Headline finding — revises the handoff hypothesis
 The handoff named the **context shims** (`context.rs`: two `call`s into the
 TLS sp accessors per switch) as the prime suspect for the per-switch cost.
 **At N=1 that is not where the time goes — the shims + TLS are ~1% of
 self-time.** The N=1 cost is dominated by:
 1. **`schedule_loop` + run-queue ops (~33%)** — the epoch/slot-word transition
   and the mutex run-queue push/pop on every re-queue.
 2. **A `futex_wake` syscall (~12%)** firing on the hot path even though nothing
   is parked. This is the **submit-rule wake** the handoff itself flagged (RFC
   004 finding #1: "a parallelism/latency optimisation, NOT a liveness guard").
   In a single-scheduler always-runnable loop it is pure cost — no one is ever
   parked to wake. **First cheap-win candidate: suppress the submit wake when
   there is no parked waiter** (a parked-count / n_spinning check before the
   syscall). Needs care — finding #2's AllDone broadcast is the liveness-critical
   one and must NOT be touched.
 ## What this does and does NOT show
 - The handoff's shim hypothesis was a **many-core** hypothesis: its evidence was
  the N=1→N=8 jump (0.18→1.2µs), attributed to TLS access mode (`__tls_get_addr`
  vs `#[thread_local]`) and cross-core coherency on the sp/epoch words. **None of
  that is observable at N=1 on one core.** This profile does NOT refute it; it
  establishes that the shim is cheap *until cores contend*.
 - So the spike question sharpens into two separable costs:
  - **N=1 floor:** scheduler logic + a likely-removable futex_wake. Actionable now.
  - **N→8 slope:** the shim/TLS/coherency cost. Needs the many-core box + a
    `remote` bench mode (wake straddling two schedulers) + hardware PMU counters
    (cache-misses, `MEM_LOAD…HITM` for coherency) — none available in this sandbox.
 ## Reproduce
 ```sh
 . "$HOME/.cargo/env"
 cargo build --release --bench switch_cost
 BIN=$(ls -t target/release/deps/switch_cost-* | grep -v '\.d$' | head -1)
 PERF=/usr/lib/linux-tools-6.8.0-124/perf   # 6.8 perf on 6.18 kernel; sw events only here
 # bench alone (numbers):
 SMARM_SWITCH_ROUNDS=3000000 SMARM_SWITCH_WARMUP=50000 SMARM_SWITCH_RUNS=4 "$BIN"
 # attribution (sw task-clock; HW counters need a real PMU / the 5900X):
 SMARM_SWITCH_ROUNDS=3000000 "$PERF" record -F 4000 -g --call-graph fp -o /tmp/switch.data -- "$BIN"
 "$PERF" report -i /tmp/switch.data --stdio --no-children
 ```
 On the 5900X with a real PMU, drop `-e task-clock` for `-e cycles,instructions,
 cache-misses,mem_load_retired.l3_miss` to get the coherency picture the N=8 case
 needs.