perf: a measurement pane is priced by its window, and its fold is inline

A pane is recomputed over the whole visible window on a cadence, and at a 15 s window
(11.8 M samples, 184 k level-0 level buckets, `--buffer-mb 4000` so the stride stays 1)
that is priced per *measured channel* and per *distinct share*. Measured with the new
`[levels-probe]` sweep, per recompute: one window with one level 6.58 ms, four shares
10.28 ms, sixteen shares 24.55 ms, and four measurements one share each over four
channels **35.69 ms** -- against a tick mean of 1.34 ms with an empty pane and 4.58 ms
with that one. That is the reported "very slow with multiple measurements": the pass
runs inside `tick()`, so it is a *frame* cost, and not an ingest one -- `gaps` and
`packets` stay clean even though each `MeasureWindowReader` holds the model mutex for
its whole life, because a socket buffer absorbs a few tens of milliseconds of producer.

The largest single term inside a pass was a call. `measure_accumulate_bucket()` folds
one bucket into one level, and it is the innermost loop of a walk whose iteration count
is the window's buckets *times* the pane's levels; it lived out of line in
`measure.cpp`, so every one of those iterations was a cross-translation-unit call.
Moving it, and the crossing state it updates, into `measure.hpp` as `inline` takes
sixteen shares on one channel from 24.55 to **17.25 ms** a recompute -- with every value
in the dump's `measurements:` unchanged. Two levers remain and both are measured: the
fetch's three out-of-line pyramid accessors plus a 48-byte copy per bucket (3.3 ms of an
11.8 M-sample window's 11.6 ms pass) needs a combined accessor upstream, and `raw` is
the *signal's* share (4.6 % on the mock's noisy channel against 0.1 % on its sine),
because a level inside a bucket's own `[lo, hi]` has to be walked sample by sample.

Bisected with the pane populated (the earlier bisect had it empty, which cannot see
this): the parent of the recent range and HEAD read **8.29** and **7.92 ms** a
recompute for the same four-type pane at the same window, and neither of the two
commits between them touches `ui_measure.cpp` or `measure.cpp` at all. So the cost is
the scale and not a regression, and the fix makes a pass cheaper rather than removing
one -- the plot's own per-frame cost was checked the same way (four commits, equal
within 3 %, and unchanged by the window, the ring size and the visible sample count).

`UDPSCOPE_PROBE_LEVELS_MS` with `UDPSCOPE_PROBE_LEVELS` and
`UDPSCOPE_PROBE_LEVELS_CHANNELS` is the sweep; `docs/claude/invariants-measurements.md`
records the curve and the inline rule, `verification.md` and `probe-readings.md` the
probe and what its numbers must read, and the README the pane's cost at this scale.

README.md

@@ -1638,6 +1638,35 @@ Measured on the same stream and window: tick mean **1.39 ms** with the pane hold
 measurements against **0.73 ms** without (595 frames against 610), the pass itself 11 ms every
 400 ms — and the 82 ms worst tick is the *control's* too, i.e. the publish stall, not this.
 
+**...and the same pane costs what the *window* costs, which a big time scale buys.** The three
+fixes above bound *how often* and *how much per sample*; what is left is the arithmetic of the
+scale: a pass is priced per measured **channel** (one fetch of the window's level-0 buckets, one
+statistics scan) and per **distinct share** in the pane (one level accumulator, walked over those
+buckets), and the window's own sample count multiplies both. Measured with the new
+`[levels-probe]` sweep on the mock at a **15 s** window and `--buffer-mb 4000` (so `time_stride`
+stays 1 and the window holds 11.8 M samples, 184 k buckets), four signals, one row, on llvmpipe
+— `measure cost:`'s `each` per recompute, and the tick's mean:
+
+| pane | `each` | `raw` | tick mean |
+|---|---|---|---|
+| empty | — | — | 1.34 ms |
+| 1 measurement, 1 level, 1 channel | 6.58 ms | 0.1 % | 1.99 ms |
+| 4 measurements, 4 shares, 1 channel | 10.28 ms | 0.4 % | 2.38 ms |
+| 16 measurements, 16 shares, 1 channel | 24.55 ms | 1.2 % | 3.86 ms |
+| 4 measurements, 1 share each, **4 channels** | 35.69 ms | 2.6 % | 4.58 ms |
+
+So a machine's ten measured signals at a 15 s window is tens of milliseconds a pass, several
+times a second, on the **tick** — the pane is a *frame* cost and not an ingest one (`gaps` and
+`packets` stay clean even though each reader holds the model mutex, which is what "the plot is
+slow with measurements on" actually is). One of those terms was a pure call-overhead defect:
+pass two folds each bucket into each level through `measure_accumulate_bucket()`, the innermost
+loop of a walk whose iteration count is the window's buckets *times* the pane's levels, and as an
+out-of-line function in its own translation unit it cost **30 % of the pass** — sixteen shares on
+one channel read `24.55 ms` a recompute with every value in `measurements:` unchanged at
+**17.25 ms** once it was moved into `measure.hpp` as `inline`. The fetch's three out-of-line
+pyramid accessors per bucket (3.3 ms of an 11.8 M-sample window's 11.6 ms pass) are the next
+lever, and `raw` is the last one and belongs to the signal rather than to the pane.
+
 **"Show on plot" draws full-width rulers, and they must not blink.** A level measurement draws an
 `h_ruler` across the whole plot and a time one a full-height cursor, so a pane holding four
 measurements can put six lines over the data — that is the feature, and the lines move with the

docs/claude/invariants-measurements.md

@@ -118,6 +118,47 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
     the pyramid for nothing.
   `measure cost:`'s `covered`/`buckets`/`raw` is the guard, and it needs the fast mock to say
   anything.
+- **A pass costs the window's buckets times the pane's *measured channels* and its *distinct
+  levels*, and the window's own size multiplies all of it.** The cadence above bounds *how
+  often*; this is *how much*, and it is the term a big time scale buys. Measured on the mock at
+  the user's scale -- a **15 s** window, `--buffer-mb 4000` (so `time_stride` stays 1 and the
+  window is 11.8 M samples, 184 k level-0 buckets), four signals, one row -- with
+  `[levels-probe]` (see [verification.md](verification.md)), `measure cost:` reading
+  `recomputes`/`each`/`covered`/`buckets`/`raw`:
+
+  | pane | `each` | `raw` | tick mean |
+  |---|---|---|---|
+  | empty | -- | -- | 1.34 ms |
+  | 1 measurement, 1 level | 6.58 ms | 0.1 % | 1.99 ms |
+  | 4 measurements, 4 shares on **one** channel | 10.28 ms | 0.4 % | 2.38 ms |
+  | 16 measurements, 16 shares on one channel | 24.55 ms | 1.2 % | 3.86 ms |
+  | 4 measurements, 1 share each on **four** channels | 35.69 ms | 2.6 % | 4.58 ms |
+
+  So a pane is priced per measured **channel** (one `MeasureWindowReader`, one fetch, one scan)
+  and per **distinct share** (one accumulator), and the window's samples are the multiplier on
+  both: the same pane over a 2 M-sample window is the 1.2--1.6 ms a recompute this pane's
+  budget was fixed at, and a machine's ten noisy channels at a 15 s window is **tens of ms a
+  pass**, several times a second. That lands on the **tick** -- `update_measures()` runs inside
+  it -- so `frame cost:`'s tick mean is where a pane's cost shows (1.34 -> 4.58 ms above), and
+  it is *not* an ingest problem: `gaps=`/`packets=` stay exactly clean even though each
+  `MeasureWindowReader` holds `model->mutex()` for its whole life, because a socket buffer
+  absorbs a few tens of ms of producer. Read the tick mean and `each` before blaming the
+  producer.
+  Inside a pass the terms are, instrumented per window at that scale: the **fetch** 3.3 ms
+  (three out-of-line `MinMaxPyramid` accessors and a 48-byte `MeasureBucket` per bucket),
+  `measure_scan_buckets()` 0.8 ms, and **pass two** 7.5 ms -- dominated by the per-(bucket,
+  level) fold. That fold is why `measure_accumulate_bucket()` is `inline` in `measure.hpp`: it
+  is the innermost loop of a walk whose iteration count is the window's buckets *times* the
+  pane's levels, and as an out-of-line call in its own translation unit it cost **30 % of the
+  pass** (sixteen shares on one channel: `24.55 -> 17.25 ms` a recompute). A caller adding a
+  per-bucket type keeps it in the header, and a caller adding a bulk one may not let it grow a
+  call per bucket either.
+  The third term is `raw`, and it is the **signal's**: a level that falls inside a bucket's
+  `[lo, hi]` has to be walked sample by sample (the fold cannot split a bucket), so a signal
+  whose local range covers the level everywhere -- white noise, a noisy machine channel --
+  degrades the pass to O(window) *per level*: measured `raw=4.6 %` on the mock's `noise` channel
+  against `0.1 %` on its `sine` at the same window. It is the field to read before blaming the
+  pane for a slow run, and no cheaper walk fixes it.
 - **`Top`/`Bottom`/`Amplitude` read a *flat* level, and the density that finds it is the buckets',
   not the samples'.** A band-limited edge overshoots and rings -- the mock's `square` at 30
   harmonics reaches ±1.179 where its plateau is ±1 -- so a measurement that reads the extremes, or

docs/claude/probe-readings.md

@@ -714,6 +714,33 @@ Two lines of the self-check output are assertions in disguise:
   reported `gaps=73` with a pause against 0 without, which is what made the stall worth measuring;
   it has not reproduced and did not scale with the publish length — 79 gaps with the pyramid build
   removed against 73 with it — so it was incidental load on one run. Do not quote it as a cost.)
+- `[levels-probe] N measurement(s) over K channel(s), shares='...' -> W window(s), L level
+  walk(s)` — then the exit dump's `measure cost:` and `frame cost:` beside it. This is how a
+  pane's cost is read, and the three fields are the three factors: **`W` is the pane's price
+  for the window**, one fetch and one scan per *measured channel*; **`L` is its price for the
+  pane's shares**, one accumulator per distinct one, walked over the window's buckets; and the
+  window's samples are the multiplier on both, so a sweep at one scale says nothing about
+  another. Measured at the user's scale (15 s window, `--buffer-mb 4000` -> `stride=1`, 11.8 M
+  samples in the window = 184 k buckets, four signals, mock at ~0.5 MS/s per signal, llvmpipe),
+  with `UDPSCOPE_PROBE_LEVELS_CHANNELS` set:
+  * `50`, 1 channel -> `1 window, 1 level`, `measure cost: 226 recomputes, 6.58 ms each`, `raw=0.1%`,
+    `tick 1.99 ms` against an empty pane's **1.34**;
+  * `5,10,15,20`, 1 channel -> `4 levels`, **10.28 ms** each, `raw=0.4%`, `tick 2.38 ms`;
+  * the sixteen shares `5..80`, 1 channel -> `16 levels`, **24.55 ms** each, `raw=1.2%`, `tick 3.86 ms`;
+  * `5,10,15,20` over **4 channels** -> 4 windows and 4 levels, **35.69 ms** each, `raw=2.6%`,
+    `tick 4.58 ms`, and still `gaps=0 packets=30580` — the pane is a *tick* cost and not an
+    ingest one, which is the thing to say before blaming the producer.
+  `measure cost:`'s `each` is the pass and `covered`/`recomputes` is the mean window it ran
+  over (11.8 M samples here), so `each` is only comparable between runs at the same window and
+  the same pane; `raw` is the *signal's* share (the mock's `noise` channel reads `4.6%` against
+  its `sine`'s `0.1%` at the same window, because a level inside a bucket's own range has to be
+  walked sample by sample). A pass that reads **tens of ms** on a 15 s window with a handful of
+  measurements is the curve in
+  [invariants-measurements.md](invariants-measurements.md) and not a defect, and the same pane
+  at a 2 s window reads the 1.2--1.6 ms the pane's budget was fixed at. The guard for the walk
+  itself is the exit dump's `frame cost:`: sixteen shares on one channel read `24.55 ms` a pass
+  and `3.86 ms` of tick mean before `measure_accumulate_bucket()` was made `inline`, and
+  **17.25 ms** a pass after it, with every value in `measurements:` unchanged.
 - `trace ink: N paints checked, C collapsed, S staircase, H short, worst cover P%, worst
   drawn-vs-quoted D px over R raw paints` plus a
   `[ink] paint N <flags> mode=.. axis=[..] n=.. idx=[..) spp=.. level=.. pts=.. kept=..

docs/claude/verification.md

@@ -76,6 +76,22 @@ precedent, not an exception.
   with, so a synchronous sequence would read every position one action stale. It clicks by
   emitting `::clicked`, never `gtk_widget_activate()`, which holds a `GtkButton` for its
   250 ms `ACTIVATE_TIMEOUT` and then emits into a button the rebuild has already destroyed.
+- `[levels-probe] N measurement(s) over K channel(s), shares='...' -> W window(s), L level
+  walk(s)` (stderr, `UDPSCOPE_PROBE_LEVELS_MS`) — **prices a measurement pane**, which is what a
+  report of "it is slow with many measurements" has to become before it is a number. It adds one
+  `above` per share in `UDPSCOPE_PROBE_LEVELS` (comma separated, default `50`), spread in turn
+  over the first `UDPSCOPE_PROBE_LEVELS_CHANNELS` channels (default 1), and reports the two
+  things a count does not name: how many **windows** (one `MeasureWindowReader`, one fetch, one
+  scan per measured channel) and how many **level walks** (one accumulator per *distinct* share)
+  the pass then had to take. `50,50,50,50` on one channel is four pane rows and **one** walk;
+  `5,10,15,20` is four rows and **four**; the same four shares over four channels is four
+  windows *and* four walks. Read the `measure cost:` line it leaves in the exit dump and the
+  sweep in [invariants-measurements.md](invariants-measurements.md) — and read it against a run
+  with `UDPSCOPE_PROBE_LEVELS_MS` unset, because the pane's cost is a *difference* between two
+  tick means. **Run it at the scale being asked about**: the cost is the window's, so a 2 s
+  window prices a 15 s one at nothing (four measurements over four channels read `35.69 ms` a
+  recompute and a 4.58 ms tick mean at a 15 s window against `6.58 ms`/1.34 ms for one window at
+  the same window).
 - `[measure-probe] N measurements on 'X'` (stderr, `UDPSCOPE_PROBE_MEASURE_MS`) — adds one
   measurement of each statistical type (average, top, peak to peak, amplitude) on the first
   *channel* and forces a recompute. It must name a **channel**, not `signals().front()`: the

src/measure.cpp

@@ -14,20 +14,6 @@ namespace {
 /// in. Shared by the raw walk and the bucket walk so the two cannot disagree about what an edge
 /// records -- an interval's width is the distance from the edge that entered it to the edge that
 /// left it, which is the pairing the pulse duration averages.
-void enter_side(MeasureLevel& acc, std::size_t e, double t) {
-  const std::size_t other = 1 - e;
-  if (acc.enters[e] == 0) acc.first_enter[e] = t;
-  acc.last_enter[e] = t;
-  ++acc.enters[e];
-  if (acc.open[other]) {
-    acc.width_sum[other] += t - acc.open_at[other];
-    ++acc.widths[other];
-    acc.open[other] = false;
-  }
-  acc.open_at[e] = t;
-  acc.open[e] = true;
-}
-
 // The hysteresis band and its rationale moved to `MeasureOptions::noise_pct`: it is a per-measurement
 // setting now, and a second copy of the number here would be a second thing to keep in step.
 constexpr double kPi = 3.14159265358979323846;
@@ -240,51 +226,6 @@ void measure_accumulate(const MeasureChunk& chunk,
   }
 }
 
-bool measure_accumulate_bucket(const MeasureBucket& k,
-                               const std::function<double(std::size_t)>& time_of,
-                               MeasureLevel& acc) {
-  if (k.count == 0) return true;
-  // A non-finite sample in the bucket would make the edge's position a guess: the raw walk takes
-  // the first *finite* sample of the new side, and an aggregate cannot say which that is.
-  if (k.count != k.span) return false;
-  // The *sums* need only the bare level: strictly on one side of it, so every sample decides the
-  // same bin and none is *at* it, which is what makes the fold exact rather than an approximation.
-  const bool above = k.lo > acc.level;
-  const bool below = k.hi < acc.level;
-  if (!above && !below) return false;
-  const std::size_t e = above ? 1u : 0u;
-  // The band is deliberately *not* folded into the test above. Tightening the whole condition to
-  // `level +/- h` refuses every bucket lying inside the band -- a tenth of the amplitude either side
-  // of the mid level is where a waveform moves fastest, so those are exactly the buckets a crossing
-  // passes through -- which hands the raw walk a share of the window rather than a bucket or two per
-  // crossing, and breaks the "`raw` stays at a fraction of a percent of `covered`" invariant.
-  // Instead: a bucket on one side of the level can always be *binned*, and only a *side change*
-  // whose position is inside the bucket is refused.
-  const double top = acc.level + acc.hysteresis;
-  const double bot = acc.level - acc.hysteresis;
-  // Does the bucket reach past the band at all? One wholly inside it decides no side, exactly as a
-  // raw sample inside it decides none, so the walk state is left alone.
-  const bool reaches = above ? (k.hi > top) : (k.lo < bot);
-  const bool change = reaches && acc.have_side && e != (acc.side ? 1u : 0u);
-  // A bucket that *enters* a side while straddling its own threshold holds the crossing somewhere
-  // inside itself, and only the raw walk can say where. One wholly past the threshold holds it at
-  // the bucket's first sample, exactly, which is where the raw walk would have found it.
-  const bool fully_outside = above ? (k.lo > top) : (k.hi < bot);
-  // Nothing may be folded before this point: a refusal hands the bucket to `measure_accumulate()`,
-  // which adds the same samples again. Sums added above the refusal double-counted every opened
-  // bucket -- which read as `top` 0.614304 on a unit sine against the 2/pi a mean over a half then
-  // gave, and which is now the sums `Top`/`Bottom` fall back to on a window with no histogram, so
-  // a wrong one is a wrong answer waiting for that window.
-  if (change && !fully_outside) return false;
-  acc.sum[e] += k.sum;
-  acc.count[e] += k.count;
-  if (change) enter_side(acc, e, time_of(k.offset));
-  if (reaches) {
-    acc.side = e != 0;
-    acc.have_side = true;
-  }
-  return true;
-}
 
 MeasureFlatLevels measure_flat_levels(const MeasureStats& b, double pct) {
   MeasureFlatLevels out;
@@ -375,7 +316,6 @@ int measure_level_pcts(MeasureType type, const MeasureOptions& options, double p
   }
 }
 
-
 const char* measure_type_name(MeasureType type) {
   switch (type) {
     case MeasureType::Average: return "average";
@@ -598,5 +538,4 @@ MeasureResult compute_measure(const MeasureStats& b, const MeasureLevel* const l
   return out;
 }
 
-
 }  // namespace udpscope

src/measure.hpp

@@ -349,6 +349,66 @@ bool measure_accumulate_bucket(const MeasureBucket& bucket,
                                const std::function<double(std::size_t)>& time_of,
                                MeasureLevel& level);
 
+inline void enter_side(MeasureLevel& acc, std::size_t e, double t) {
+  const std::size_t other = 1 - e;
+  if (acc.enters[e] == 0) acc.first_enter[e] = t;
+  acc.last_enter[e] = t;
+  ++acc.enters[e];
+  if (acc.open[other]) {
+    acc.width_sum[other] += t - acc.open_at[other];
+    ++acc.widths[other];
+    acc.open[other] = false;
+  }
+  acc.open_at[e] = t;
+  acc.open[e] = true;
+}
+
+inline bool measure_accumulate_bucket(const MeasureBucket& k,
+                               const std::function<double(std::size_t)>& time_of,
+                               MeasureLevel& acc) {
+  if (k.count == 0) return true;
+  // A non-finite sample in the bucket would make the edge's position a guess: the raw walk takes
+  // the first *finite* sample of the new side, and an aggregate cannot say which that is.
+  if (k.count != k.span) return false;
+  // The *sums* need only the bare level: strictly on one side of it, so every sample decides the
+  // same bin and none is *at* it, which is what makes the fold exact rather than an approximation.
+  const bool above = k.lo > acc.level;
+  const bool below = k.hi < acc.level;
+  if (!above && !below) return false;
+  const std::size_t e = above ? 1u : 0u;
+  // The band is deliberately *not* folded into the test above. Tightening the whole condition to
+  // `level +/- h` refuses every bucket lying inside the band -- a tenth of the amplitude either side
+  // of the mid level is where a waveform moves fastest, so those are exactly the buckets a crossing
+  // passes through -- which hands the raw walk a share of the window rather than a bucket or two per
+  // crossing, and breaks the "`raw` stays at a fraction of a percent of `covered`" invariant.
+  // Instead: a bucket on one side of the level can always be *binned*, and only a *side change*
+  // whose position is inside the bucket is refused.
+  const double top = acc.level + acc.hysteresis;
+  const double bot = acc.level - acc.hysteresis;
+  // Does the bucket reach past the band at all? One wholly inside it decides no side, exactly as a
+  // raw sample inside it decides none, so the walk state is left alone.
+  const bool reaches = above ? (k.hi > top) : (k.lo < bot);
+  const bool change = reaches && acc.have_side && e != (acc.side ? 1u : 0u);
+  // A bucket that *enters* a side while straddling its own threshold holds the crossing somewhere
+  // inside itself, and only the raw walk can say where. One wholly past the threshold holds it at
+  // the bucket's first sample, exactly, which is where the raw walk would have found it.
+  const bool fully_outside = above ? (k.lo > top) : (k.hi < bot);
+  // Nothing may be folded before this point: a refusal hands the bucket to `measure_accumulate()`,
+  // which adds the same samples again. Sums added above the refusal double-counted every opened
+  // bucket -- which read as `top` 0.614304 on a unit sine against the 2/pi a mean over a half then
+  // gave, and which is now the sums `Top`/`Bottom` fall back to on a window with no histogram, so
+  // a wrong one is a wrong answer waiting for that window.
+  if (change && !fully_outside) return false;
+  acc.sum[e] += k.sum;
+  acc.count[e] += k.count;
+  if (change) enter_side(acc, e, time_of(k.offset));
+  if (reaches) {
+    acc.side = e != 0;
+    acc.have_side = true;
+  }
+  return true;
+}
+
 /// The levels a type reads, as shares of the window's own min..max, **lower first**, or 0 when it
 /// reads none (the pure statistics, answered from `MeasureStats` alone). `pct` must hold two: a
 /// level type fills `pct[0]` and answers 1, `Between` fills both and answers 2.

src/ui.cpp

@@ -5831,6 +5831,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_MEASURE_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), measure_probe, &app);
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_LEVELS_MS")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), levels_probe, &app);
+  }
   if (const char* env = g_getenv("UDPSCOPE_PROBE_MEASUREPIN_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), measurepin_probe, &app);
   }

src/ui_app.hpp

@@ -1730,6 +1730,7 @@ gboolean mathwrap_probe(gpointer user);
 gboolean mathwrap_probe_wait(gpointer user);
 gboolean mathwrap_probe_report(gpointer user);
 gboolean measure_probe(gpointer user);
+gboolean levels_probe(gpointer user);
 gboolean measurepin_probe(gpointer user);
 gboolean measurepin_held_wait(gpointer user);
 gboolean measurepin_pause_wait(gpointer user);

src/ui_probe.cpp

@@ -1326,6 +1326,71 @@ gboolean measure_probe(gpointer user) {
   return G_SOURCE_REMOVE;
 }
 
+/// `UDPSCOPE_PROBE_LEVELS_MS`: puts a pane of **N** measurements on screen -- one `above` per
+/// share in `UDPSCOPE_PROBE_LEVELS` (comma separated, default `50`) -- spread in turn over the
+/// first `UDPSCOPE_PROBE_LEVELS_CHANNELS` channels (default 1), and says how many *windows* and
+/// *levels* the pass then had to walk.
+///
+/// The pane's cost has three factors and a count alone names none of them: the window's samples
+/// (one fetch and one scan per **measured channel**, whatever the pane holds), the number of
+/// *distinct levels* in it (measurements that share a share share the walk), and the rows that
+/// are only labels over the two. `50,50,50,50` is four rows and one walk; `5,10,15,20` is four
+/// rows and four; and the same four shares on four signals is four windows *and* four walks.
+/// This probe is the only way to vary the three apart, which is what a report of "it is slow
+/// with many measurements" needs before it is a number.
+gboolean levels_probe(gpointer user) {
+  App* a = static_cast<App*>(user);
+  if (a->window == nullptr) return G_SOURCE_REMOVE;
+  std::vector<std::string> names;
+  {
+    std::lock_guard<std::mutex> lock(a->model->mutex());
+    for (const ChannelInfo& c : a->model->channels()) names.push_back(c.name);
+  }
+  if (names.empty()) return G_SOURCE_REMOVE;
+  const char* chan_env = g_getenv("UDPSCOPE_PROBE_LEVELS_CHANNELS");
+  const long want_channels = chan_env != nullptr ? std::atol(chan_env) : 1;
+  const std::size_t channels =
+      want_channels > 0 ? std::min<std::size_t>(static_cast<std::size_t>(want_channels),
+                                                names.size())
+                        : 1u;
+  const char* list = g_getenv("UDPSCOPE_PROBE_LEVELS");
+  const std::string shares = list != nullptr && list[0] != '\0' ? list : "50";
+  std::size_t rows = 0;
+  for (std::size_t at = 0; at <= shares.size();) {
+    const std::size_t comma = shares.find(',', at);
+    const std::string field =
+        shares.substr(at, comma == std::string::npos ? std::string::npos : comma - at);
+    at = comma == std::string::npos ? shares.size() + 1u : comma + 1u;
+    char* end = nullptr;
+    const double pct = std::strtod(field.c_str(), &end);
+    if (end == field.c_str() || !(pct >= 0.0)) continue;
+    Measure m;
+    m.signal = names[rows % channels];
+    m.type = MeasureType::Above;
+    m.options.threshold_pct = pct;
+    a->measures.push_back(std::move(m));
+    ++rows;
+  }
+  rebuild_measures_pane(*a);
+  update_measures(*a, /*force=*/true, /*accumulate=*/true);
+  // What that pass walked: the level accumulators the pane deduplicated to, and the windows they
+  // hang off. Read without the model lock -- `measure_samples` is the *tick*'s own state, not the
+  // producer's.
+  std::size_t levels = 0;
+  std::size_t windows = 0;
+  for (const App::MeasureWindow& w : a->measure_samples) {
+    ++windows;
+    levels += w.pct.size();
+  }
+  std::fprintf(stderr,
+               "[levels-probe] %zu measurement(s) over %zu channel(s), shares='%s' -> %zu "
+               "window(s), %zu level walk(s)\n",
+               rows, channels, shares.c_str(), windows, levels);
+  std::fflush(stderr);
+  g_timeout_add(250, measure_trace, a);
+  return G_SOURCE_REMOVE;
+}
+
 /// `UDPSCOPE_PROBE_MEASUREPIN_MS`: on a **still frame** the pane describes the frame and not the
 /// viewport, so a pan or a zoom changes no number -- and on a live view the numbers still follow
 /// their window.