feat(numa): introduce I/O sampling to prevent activation stalls

Replaces the monolithic CPU scaling threshold with separate CPU and I/O spawn thresholds. Introduces an `IoSample` struct with platform-specific byte reading and a relative throughput growth heuristic. Adds a 0.1s wall-clock guard to `CpuSample` to suppress artificial efficiency spikes, and updates `maybe_activate` to trigger worker scaling when either resource indicates headroom. Bumps `obikmer` to v1.1.33 and updates architecture documentation.

docmd/architecture/numa_partition_runner.md

@@ -162,14 +162,107 @@ A single `PartitionRunner` instance can be built once per command invocation
 and reused across multiple `run()` calls (e.g. `merge` runs
 `merge_partitions` then `pack_matrices`).
 
+## Known issue: CPU-only activation signal stalls on I/O-bound stages
+
+Observed on a real `filter` run (109 genomes, 256 partitions, 8×24-core NUMA):
+`rebuild` (CPU-bound — k-mer construction) scales cleanly from 9 to 43 active
+workers as `CpuSample::do_i_activate` (`obisys::lib.rs`) sees efficiency climb.
+`pack_matrices` (I/O-bound — reopens and recomposes per-genome column files
+into `.pbmx`/`.pcmx`) activates one extra worker then flatlines at 10/192 for
+the rest of the stage, even though 256 partitions keep completing over several
+minutes. This matches the documented intent (§ Adaptive mechanism — "avoids
+over-provisioning ... I/O-bound ... workloads") but conflates two different
+things: *"CPU is not the bottleneck"* and *"more workers would not help"*. On
+storage with real queue depth (NVMe, RAID, parallel FS) the second stage could
+still benefit from more concurrent workers even with flat CPU usage — a signal
+the current mechanism cannot see.
+
+A one-off artefact was also found in the same log: right after a stage
+transition, `do_i_activate` produced a physically impossible spike (efficiency
+~94 cores on a 192-core box) because it has no minimum-window guard — unlike
+its sibling `cpu_efficiency`, which returns `0.0` if `wall < 0.1s`
+(`obisys::lib.rs:260`). `do_i_activate` unconditionally overwrites
+`self.wall`/`self.user_secs`/`self.sys_secs` even when the elapsed window is
+too short to be meaningful, so a burst of rapid completions right after
+activating a worker can divide a real CPU delta by a near-zero wall delta.
+
+### Implemented: I/O signal + shared debounce guard
+
+`IoSample` (`obisys::lib.rs`, alongside `CpuSample`) is fed by
+`read_bytes`/`write_bytes` from `/proc/self/io` on Linux (actual bytes
+submitted to the block layer — not `rchar`/`wchar`, which also count
+page-cache hits, and not `ru_inblock`/`ru_oublock`, unreliable on macOS), with
+a `proc_pid_rusage(RUSAGE_INFO_V4)` fallback on macOS
+(`ri_diskio_bytesread`/`ri_diskio_byteswritten`, FFI only via `libc`, no new
+dependency — same pattern as the existing `getrusage` bindings). Any other
+target degrades gracefully to a signal that never triggers (falls back to
+CPU-only activation), same pattern as `cgroup_v2_available`.
+
+`maybe_activate` (`numa.rs`) activates a worker if *either* signal still shows
+headroom, making `PartitionRunner` adapt to whichever resource is actually the
+bottleneck without per-call configuration. Both samplers are called
+unconditionally — no `||` short-circuit — so neither window starves behind
+whichever signal fires first:
+
+```rust
+let cpu_wants_more = cpu_sample.do_i_activate(CPU_SPAWN_THRESHOLD);
+let io_wants_more  = io_sample.do_i_activate(IO_SPAWN_THRESHOLD);
+if cpu_wants_more || io_wants_more {
+    activate_tx.send(()).ok();
+    ...
+}
+```
+
+Unlike the CPU signal (an absolute delta in cores — a bounded, portable unit),
+raw I/O throughput has no natural scale across devices, so `IoSample` uses a
+**relative** growth threshold instead of an absolute one:
+
+```rust
+pub fn do_i_activate(&mut self, threshold: f64) -> bool {
+    let elapsed = self.wall.elapsed().as_secs_f64();
+    if elapsed < 0.1 { return false; }        // state untouched — window keeps accumulating
+
+    let n = Self::read_bytes();
+    let rate = n.saturating_sub(self.bytes) as f64 / elapsed;
+    let activate = if self.previous_rate == 0.0 {
+        rate > 0.0                            // bootstrap: any measured throughput is signal
+    } else {
+        (rate - self.previous_rate) / self.previous_rate >= threshold
+    };
+
+    self.bytes = n;
+    self.wall  = Instant::now();              // reset only on a real sample
+    activate
+}
+```
+
+The `elapsed < 0.1s → return false without mutating state` guard was also
+back-ported into `CpuSample::do_i_activate` (previously missing — source of
+the ~94-core artefact above) — one fix for both problems, and it removes the
+need for any arbitrary I/O-rate floor: a short/noisy window is rejected
+outright rather than papered over with a hardware-dependent constant.
+
+Both spawn thresholds (`CPU_SPAWN_THRESHOLD`, `IO_SPAWN_THRESHOLD`, both `0.2`)
+are defined as `const` in `PartitionRunner::run` (`numa.rs`). The I/O value is
+a starting point, not a derived one — needs empirical validation against a
+real `pack` run.
+
+Starting threshold: `0.2` (20 % relative growth) for `IoSample`, same order of
+magnitude as the CPU threshold's *implicit* relative sensitivity (in the
+observed log, an 8→9 worker step raised efficiency by ~12 %). This is a
+starting point, not a derived value — I/O throughput is lumpier than CPU time
+(buffered writes flush in bursts), so it needs empirical validation against a
+real `pack` run before being considered final.
+
 ## Open questions
 
 - **Error handling**: `run` currently returns the first error; remaining errors
   are dropped.  A `Vec<E>` return would give complete diagnostics.
 
 - **`workers_per_node` tuning**: currently `(cpus / 8).max(3).min(8)`, calibrated
-  for merge on BeeGFS.  I/O-bound commands (`dump`, `select`) may benefit from
-  a higher value.  A per-call override could be added to the API.
+  for merge on BeeGFS.  Superseded by the I/O signal above for the "more
+  workers would help despite flat CPU" case — a per-call override may still be
+  worth keeping as a manual escape hatch.
 
 - **`on_done` ordering**: the runner serialises calls to `on_done` via an
   internal `Arc<Mutex<C>>`.  `Send` is required (the Arc clone crosses thread

src/Cargo.lock

@@ -1704,7 +1704,7 @@ dependencies = [
 
 [[package]]
 name = "obikmer"
-version = "1.1.30"
+version = "1.1.33"
 dependencies = [
  "clap",
  "csv",

src/obikindex/src/numa.rs

@@ -20,7 +20,7 @@ use hwlocality::cpu::binding::CpuBindingFlags;
 use hwlocality::cpu::cpuset::CpuSet;
 #[cfg(feature = "numa")]
 use hwlocality::object::types::ObjectType;
-use obisys::CpuSample;
+use obisys::{CpuSample, IoSample};
 use tracing::debug;
 
 // ── Public interface ──────────────────────────────────────────────────────────
@@ -190,10 +190,13 @@ impl PartitionRunner {
     /// Run `f(i)` for every index in `order`.
     ///
     /// Workers are pre-spawned dormant and activated adaptively. A timer thread
-    /// fires a CPU-efficiency check every `TIMER_SECS` seconds; each completed
+    /// fires an efficiency check every `TIMER_SECS` seconds; each completed
     /// partition resets that timer (forcing an immediate check) and also
-    /// triggers its own inline check. A new worker is activated whenever
-    /// efficiency falls below `SPAWN_THRESHOLD`.
+    /// triggers its own inline check. A new worker is activated whenever CPU
+    /// efficiency grows by at least `CPU_SPAWN_THRESHOLD` (absolute, in cores)
+    /// or I/O throughput grows by at least `IO_SPAWN_THRESHOLD` (relative) since
+    /// the last check — whichever resource is the actual bottleneck still shows
+    /// headroom.
     ///
     /// `on_done(i, result, elapsed)` is called from the controller thread as
     /// each partition completes — suitable for progress bars and result
@@ -217,7 +220,8 @@ impl PartitionRunner {
             return Ok(());
         }
 
-        const SPAWN_THRESHOLD: f64 = 0.2;
+        const CPU_SPAWN_THRESHOLD: f64 = 0.2;
+        const IO_SPAWN_THRESHOLD:  f64 = 0.2;
         const TIMER_SECS:          u64 = 30;
 
         // ── Channels ──────────────────────────────────────────────────────────
@@ -287,6 +291,7 @@ impl PartitionRunner {
             for _ in 0..initial_workers { activate_tx.send(()).ok(); }
             let mut n_active   = initial_workers;
             let mut cpu_sample = CpuSample::now();
+            let mut io_sample  = IoSample::now();
             let mut completed  = 0usize;
 
             while completed < n_total {
@@ -303,13 +308,17 @@ impl PartitionRunner {
                         // Inline check: same logic as a timer tick.
                         maybe_activate(
                             &activate_tx, &mut n_active, max_workers,
-                            &mut cpu_sample, SPAWN_THRESHOLD, completed, n_total,
+                            &mut cpu_sample, CPU_SPAWN_THRESHOLD,
+                            &mut io_sample, IO_SPAWN_THRESHOLD,
+                            completed, n_total,
                         );
                     }
                     WorkerEvent::TimerTick => {
                         maybe_activate(
                             &activate_tx, &mut n_active, max_workers,
-                            &mut cpu_sample, SPAWN_THRESHOLD, completed, n_total,
+                            &mut cpu_sample, CPU_SPAWN_THRESHOLD,
+                            &mut io_sample, IO_SPAWN_THRESHOLD,
+                            completed, n_total,
                         );
                     }
                 }
@@ -340,13 +349,21 @@ fn maybe_activate(
     n_active:      &mut usize,
     max_workers:   usize,
     cpu_sample:    &mut CpuSample,
-    threshold:   f64,
+    cpu_threshold: f64,
+    io_sample:     &mut IoSample,
+    io_threshold:  f64,
     completed:     usize,
     n_total:       usize,
 ) {
     if *n_active >= max_workers || completed >= n_total { return; }
 
-    if cpu_sample.do_i_activate(threshold) {
+    // Call both unconditionally (no `||` short-circuit): each sampler must
+    // advance its own window every tick, regardless of what the other one
+    // reports, or it would starve behind whichever signal fires first.
+    let cpu_wants_more = cpu_sample.do_i_activate(cpu_threshold);
+    let io_wants_more  = io_sample.do_i_activate(io_threshold);
+
+    if cpu_wants_more || io_wants_more {
         activate_tx.send(()).ok();
         *n_active += 1;
         debug!("activated worker {}/{}", n_active, max_workers);

src/obikmer/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "obikmer"
-version = "1.1.30"
+version = "1.1.33"
 edition = "2024"
 
 [[bin]]

src/obisys/src/lib.rs

@@ -266,14 +266,19 @@ impl CpuSample {
     }
 
     pub fn do_i_activate(&mut self, threshold: f64) -> bool {
+        let delta_wall = self.wall.elapsed().as_secs_f64();
+        if delta_wall < 0.1 {
+            // Window too short to be meaningful — leave state untouched so it
+            // keeps accumulating until a real sample can be taken.
+            return false;
+        }
+
         let n = CpuSample::now();
         let delta_ru = (n.user_secs - self.user_secs) + (n.sys_secs - self.sys_secs);
-        let delta_wall = self.wall.elapsed().as_secs_f64();
 
         let efficiency = delta_ru / delta_wall;
         let activate = 0f64.max(efficiency - self.previous) >= threshold;
 
-        if activate {
         debug!(
             "Do I activate : {} -> {} = {} Activate: {}",
             self.previous,
@@ -285,7 +290,93 @@ impl CpuSample {
         self.user_secs = n.user_secs;
         self.sys_secs = n.sys_secs;
         self.wall = n.wall;
+
+        activate
     }
+}
+
+// ── IoSample ──────────────────────────────────────────────────────────────────
+
+/// Snapshot of process-wide block I/O (bytes read + written) + wall clock.
+///
+/// Same activation protocol as [`CpuSample`], but the growth check in
+/// [`do_i_activate`](Self::do_i_activate) is *relative* rather than absolute:
+/// raw I/O throughput has no portable scale across storage devices, unlike a
+/// core count.
+pub struct IoSample {
+    wall: Instant,
+    bytes: u64,
+    previous_rate: f64,
+}
+
+impl IoSample {
+    pub fn now() -> Self {
+        Self {
+            wall: Instant::now(),
+            bytes: Self::read_bytes(),
+            previous_rate: 0.0,
+        }
+    }
+
+    /// Bytes actually submitted to the block layer (read + write), summed
+    /// process-wide. Returns 0 if unavailable — degrades gracefully to a
+    /// signal that never triggers activation (CPU-only heuristic).
+    #[cfg(target_os = "linux")]
+    fn read_bytes() -> u64 {
+        let Ok(io) = std::fs::read_to_string("/proc/self/io") else {
+            return 0;
+        };
+        io.lines()
+            .filter_map(|l| {
+                l.strip_prefix("read_bytes: ")
+                    .or_else(|| l.strip_prefix("write_bytes: "))
+            })
+            .filter_map(|v| v.trim().parse::<u64>().ok())
+            .sum()
+    }
+
+    #[cfg(target_os = "macos")]
+    fn read_bytes() -> u64 {
+        use libc::{RUSAGE_INFO_V4, getpid, proc_pid_rusage, rusage_info_v4};
+        let mut info: rusage_info_v4 = unsafe { std::mem::zeroed() };
+        let ret =
+            unsafe { proc_pid_rusage(getpid(), RUSAGE_INFO_V4, &mut info as *mut _ as *mut _) };
+        if ret != 0 {
+            return 0;
+        }
+        info.ri_diskio_bytesread + info.ri_diskio_byteswritten
+    }
+
+    #[cfg(not(any(target_os = "linux", target_os = "macos")))]
+    fn read_bytes() -> u64 {
+        0
+    }
+
+    /// Same protocol as [`CpuSample::do_i_activate`] (0.1 s minimum window,
+    /// state untouched on early return), but growth is measured relative to
+    /// the previous rate. `threshold` is a fraction, e.g. `0.2` for a 20 %
+    /// increase in throughput since the last real sample.
+    pub fn do_i_activate(&mut self, threshold: f64) -> bool {
+        let elapsed = self.wall.elapsed().as_secs_f64();
+        if elapsed < 0.1 {
+            return false;
+        }
+
+        let n = Self::read_bytes();
+        let rate = n.saturating_sub(self.bytes) as f64 / elapsed;
+        let activate = if self.previous_rate == 0.0 {
+            rate > 0.0 // bootstrap: any measured throughput is signal enough
+        } else {
+            (rate - self.previous_rate) / self.previous_rate >= threshold
+        };
+
+        debug!(
+            "Do I activate (I/O) : {} -> {} Activate: {}",
+            self.previous_rate, rate, activate
+        );
+        self.previous_rate = rate;
+        self.bytes = n;
+        self.wall = Instant::now();
 
         activate
     }