bench: add switch_cost local-mode per-switch microbench (rdtsc+wall, RFC per-switch spike)
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smarm-agent
@@ -65,3 +65,7 @@ harness = false
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[[bench]]
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name = "spin_sweep"
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harness = false
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[[bench]]
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name = "switch_cost"
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harness = false
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@@ -0,0 +1,256 @@
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//! Per-switch (context-switch) cost microbench — the profiling-spike harness
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//! for the ROADMAP "Per-switch cost (context shims, epoch protocol)" item.
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//!
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//! The spin work (RFC 004) is closed; the next perf target is the per-switch
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//! cost itself. Shootout evidence: per-wake latency is ~0.16–0.18µs at N=1 but
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//! ~0.8–1.2µs at N=8+, and the residual is attributed to the context-switch
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//! shims (`src/context.rs`) and the epoch protocol — NOT the queue. This binary
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//! isolates that round-trip so the cycles can be attributed under `perf` and an
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//! rdtsc bracket, feeding the RFC.
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//!
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//! WHAT THE ROUND-TRIP IS
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//!
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//! `yield_now()` from inside an actor does exactly one park/unpark round-trip
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//! with nothing else attached:
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//!
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//! actor: switch_to_scheduler ──► scheduler re-queues the actor (slot-word
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//! (context.rs shim) epoch/state transition, run_queue push),
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//! pops it straight back, switch_to_actor
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//! actor resumes ◄──────────────────────────────────────────────────────
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//!
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//! No IO thread traffic, no channel, no timer, no cross-thread wake. On a
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//! single-scheduler runtime the re-queue+repop never leaves this core, so the
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//! sample is the *pure* shim + epoch + queue-op cost with zero coherency
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//! traffic. That is the `local` baseline; a `remote` mode (wake straddling two
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//! schedulers, to expose the N=1→N=8 coherency/TLS-mode jump) is a deliberate
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//! follow-up and is NOT in this file yet — local first, per the spike plan.
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//!
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//! TWO LENSES ON THE SAME LOOP
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//!
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//! wall — `Instant` bracket per round-trip. Source of truth for µs, directly
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//! comparable to the shootout's per-wake latency numbers.
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//! cycles — `rdtsc` bracket per round-trip. Source of truth for the cycle
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//! budget the RFC will reason in (the spin budget is in cycles too).
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//!
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//! Reporting both lets us *derive* the effective TSC frequency (cycles/ns) from
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//! the same samples instead of hardcoding a nominal 3.7GHz — the spin_sweep
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//! lesson was that nominal-vs-actual TSC drift is exactly what produces
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//! red-herring numbers. If the derived freq matches the box's known base clock,
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//! the two lenses corroborate; if not, that mismatch is itself a finding.
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//!
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//! Knobs (env):
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//! SMARM_SWITCH_ROUNDS round-trips timed per run default 200000
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//! SMARM_SWITCH_WARMUP untimed warmup round-trips default 10000
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//! SMARM_SWITCH_RUNS runs (pooled latency, median) default 5
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//!
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//! Output: house table + one greppable line per run-set:
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//! SWITCHCSV,<variant>,<mode>,<rounds>,<runs>,<n>,<p50_ns>,<p90_ns>,<p99_ns>,
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//! <min_ns>,<max_ns>,<mean_ns>,<mean_cyc>,<derived_ghz>
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//!
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//! NOTE: a single yielding actor is the cooperative-scheduling tightest loop —
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//! it never parks on a futex (the work is always immediately re-queued), so
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//! this measures the switch+epoch+queue path, NOT the futex park. That is
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//! intentional: the futex park is the spin work's territory (RFC 004), already
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//! characterised. The unattributed constant the shootout flagged lives in the
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//! switch itself, which is what this loop hammers.
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use smarm::runtime::{init, Config};
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use smarm::{run, spawn, yield_now};
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use std::sync::{Arc, Mutex};
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// --------------------------------------------------------------------------
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// env helpers (house style, matching spin_sweep.rs / rq_runtime.rs)
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// --------------------------------------------------------------------------
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fn variant() -> &'static str {
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if cfg!(feature = "rq-mpmc") {
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"rq-mpmc"
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} else if cfg!(feature = "rq-striped") {
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"rq-striped"
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} else {
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"rq-mutex"
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}
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}
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fn env_usize(key: &str, default: usize) -> usize {
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std::env::var(key).ok().and_then(|v| v.parse().ok()).unwrap_or(default)
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}
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// --------------------------------------------------------------------------
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// rdtsc — serialised so the bracket actually fences the round-trip.
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//
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// Plain `rdtsc` can be reordered around the work by an out-of-order core, which
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// would smear the bracket. `rdtscp` retires prior instructions before reading
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// the counter, and the trailing `lfence` blocks later instructions from
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// climbing above the second read. Pair = (rdtscp; lfence) … work … (rdtscp;
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// lfence): a standard cycle-accurate bracket. We read TSC_AUX too but ignore
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// it; the point is the ordering guarantee, not the core id.
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// --------------------------------------------------------------------------
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#[inline(always)]
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fn rdtsc_serialised() -> u64 {
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#[cfg(target_arch = "x86_64")]
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unsafe {
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let mut aux = 0u32;
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let t = core::arch::x86_64::__rdtscp(&mut aux);
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core::arch::x86_64::_mm_lfence();
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t
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}
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#[cfg(not(target_arch = "x86_64"))]
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{
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// Non-x86 fallback: nanosecond clock standing in for cycles. The derived
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// "GHz" column then reads ~1.0 and is meaningless, but the wall lens and
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// the harness still work. The spike target box is x86-64.
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use std::time::Instant;
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thread_local! { static T0: Instant = Instant::now(); }
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T0.with(|t0| t0.elapsed().as_nanos() as u64)
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}
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}
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// --------------------------------------------------------------------------
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// percentile / median helpers (verbatim house idiom from spin_sweep.rs)
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// --------------------------------------------------------------------------
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/// Nearest-rank percentile over an already-sorted slice. `p` in [0, 100].
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fn pct(sorted: &[u64], p: f64) -> u64 {
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if sorted.is_empty() {
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return 0;
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}
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let idx = ((p / 100.0) * (sorted.len() - 1) as f64).round() as usize;
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sorted[idx.min(sorted.len() - 1)]
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}
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// --------------------------------------------------------------------------
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// one run: a single actor yields ROUNDS times; we bracket each yield from
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// inside the actor (the only vantage point — the actor is suspended during the
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// scheduler half, so an external timer can't see a single round-trip).
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//
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// Per iteration we capture BOTH a wall-ns delta and a TSC-cycle delta around
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// the same `yield_now()`. The loop overhead (two clock reads + a Vec push +
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// the branch) rides along in every sample equally; we subtract an empty-loop
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// self-calibration below so the reported number is the round-trip, not the
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// instrumentation.
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// --------------------------------------------------------------------------
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struct RunSample {
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lat_ns: Vec<u64>,
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cyc: Vec<u64>,
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}
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fn one_run(threads: usize, rounds: usize, warmup: usize) -> RunSample {
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let out: Arc<Mutex<Option<RunSample>>> = Arc::new(Mutex::new(None));
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let out2 = out.clone();
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let cfg = Config::exact(threads);
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init(cfg);
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run(move || {
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let h = spawn(move || {
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// Warmup: let the actor's stack/queue slot go hot, JIT-free but
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// cache-warm, before any sample is kept.
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for _ in 0..warmup {
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yield_now();
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}
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let mut lat_ns = Vec::with_capacity(rounds);
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let mut cyc = Vec::with_capacity(rounds);
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for _ in 0..rounds {
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let w0 = std::time::Instant::now();
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let c0 = rdtsc_serialised();
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yield_now();
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let c1 = rdtsc_serialised();
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let w1 = w0.elapsed();
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cyc.push(c1.saturating_sub(c0));
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lat_ns.push(w1.as_nanos() as u64);
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}
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*out2.lock().unwrap() = Some(RunSample { lat_ns, cyc });
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});
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let _ = h.join();
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});
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let sample = out.lock().unwrap().take().expect("actor stored a sample");
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sample
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}
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/// Empty-loop self-calibration: the same bracket with the `yield_now()` removed,
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/// run inline (no runtime). Gives the floor cost of two serialised clock reads +
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/// the push, in both lenses, to subtract from the round-trip samples.
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fn calibrate(rounds: usize) -> (u64, u64) {
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let mut lat_ns = Vec::with_capacity(rounds);
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let mut cyc = Vec::with_capacity(rounds);
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let mut sink = 0u64;
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for _ in 0..rounds {
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let w0 = std::time::Instant::now();
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let c0 = rdtsc_serialised();
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// no yield — measure the bracket itself
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let c1 = rdtsc_serialised();
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let w1 = w0.elapsed();
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sink ^= c1;
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cyc.push(c1.saturating_sub(c0));
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lat_ns.push(w1.as_nanos() as u64);
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}
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std::hint::black_box(sink);
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cyc.sort_unstable();
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lat_ns.sort_unstable();
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// Use the medians as the floor — robust to the occasional interrupt.
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(pct(&lat_ns, 50.0), pct(&cyc, 50.0))
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}
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fn main() {
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let rounds = env_usize("SMARM_SWITCH_ROUNDS", 200_000);
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let warmup = env_usize("SMARM_SWITCH_WARMUP", 10_000);
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let runs = env_usize("SMARM_SWITCH_RUNS", 5);
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let mode = "local";
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// Calibrate the instrumentation floor once, with a healthy sample.
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let (floor_ns, floor_cyc) = calibrate(rounds.min(50_000).max(10_000));
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let mut pooled_ns: Vec<u64> = Vec::new();
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let mut pooled_cyc: Vec<u64> = Vec::new();
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for _ in 0..runs {
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let s = one_run(1, rounds, warmup);
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// Subtract the instrumentation floor; saturating so a sub-floor outlier
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// (clock granularity) clamps to 0 rather than wrapping.
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pooled_ns.extend(s.lat_ns.iter().map(|&v| v.saturating_sub(floor_ns)));
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pooled_cyc.extend(s.cyc.iter().map(|&v| v.saturating_sub(floor_cyc)));
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}
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pooled_ns.sort_unstable();
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pooled_cyc.sort_unstable();
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let n = pooled_ns.len();
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let mean_ns = pooled_ns.iter().map(|&v| v as f64).sum::<f64>() / n.max(1) as f64;
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let mean_cyc = pooled_cyc.iter().map(|&v| v as f64).sum::<f64>() / n.max(1) as f64;
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// Derived effective frequency: cycles per ns = GHz. Cross-checks the two
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// lenses against the box's known base clock.
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let derived_ghz = if mean_ns > 0.0 { mean_cyc / mean_ns } else { 0.0 };
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let p50 = pct(&pooled_ns, 50.0);
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let p90 = pct(&pooled_ns, 90.0);
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let p99 = pct(&pooled_ns, 99.0);
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let lo = *pooled_ns.first().unwrap_or(&0);
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let hi = *pooled_ns.last().unwrap_or(&0);
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// House table.
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println!();
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println!("per-switch cost — {} mode, variant={}", mode, variant());
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println!(
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" rounds={} warmup={} runs={} (instrumentation floor: {} ns / {} cyc, subtracted)",
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rounds, warmup, runs, floor_ns, floor_cyc
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);
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println!(" {:<10} {:<10} {:<10} {:<10} {:<10}", "p50 ns", "p90 ns", "p99 ns", "min ns", "max ns");
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println!(" {:<10} {:<10} {:<10} {:<10} {:<10}", p50, p90, p99, lo, hi);
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println!(
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" mean {:.1} ns | mean {:.0} cyc | derived {:.3} GHz",
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mean_ns, mean_cyc, derived_ghz
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);
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// Greppable line — same spirit as SPINCSV.
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println!(
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"SWITCHCSV,{},{},{},{},{},{},{},{},{},{},{:.1},{:.0},{:.3}",
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variant(), mode, rounds, runs, n, p50, p90, p99, lo, hi, mean_ns, mean_cyc, derived_ghz
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);
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}
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