# The MEASUREMENT PROBE, against 499d2bd5f. It is banked here and NOT in the
# tree on purpose: one release binary per box serves the whole ladder, which
# is what makes the rungs comparable, and the knob must never land.
@@ -93,6 +93,18 @@
 /// before it; two BLAKE3 threads outrun one MD5 chain on every SIMD class
 /// in the fleet.
 const ENROL_THREADS: usize = 2;
+
+/// MEASUREMENT PROBE, claim `digest-cache-enrol-threads-gate-16sep`.
+/// MUST NEVER LAND: it exists so one release binary can serve a whole
+/// `ENROL_THREADS` ladder without a rebuild per rung, which is what
+/// makes the rungs comparable at all.
+fn enrol_threads_probe() -> usize {
+    std::env::var("PARFAST_ENROL_THREADS_PROBE")
+        .ok()
+        .and_then(|v| v.parse::<usize>().ok())
+        .filter(|n| *n >= 1)
+        .unwrap_or(ENROL_THREADS)
+}
 
 /// A store of validated digest records, at one directory.
 pub struct DigestCache {
@@ -669,7 +681,7 @@
         let threads = if record.is_some() {
             VALIDATE_THREADS
         } else {
-            ENROL_THREADS
+            enrol_threads_probe()
         }
         .min(machine);
         let file = Arc::new(file);