Answer 511 for an empty event-loop-delay histogram's percentiles

Node creates monitorEventLoopDelay's histogram with a lowest discernible
value of 1000 ns, so an empty histogram's percentile() returns 511, the
highest value its first bucket holds. The runtime returned 0.

6397-event-loop-metrics.ts now skips a sampling window that has no
recorded delta. A delta needs two samples after reset(), and a loop that
wakes late runs only one sample in a window, so on a loaded machine the
program printed an empty window under Node and compiled alike. The
program also prints percentile(50) after each reset.

packages/runtime/src/scr_perf_loop.c

@@ -274,10 +274,16 @@ void scr_eld_reset(double handle) {
   if (h != NULL) scr_eld_clear(h);
 }
 
+/* Node's event-loop-delay histogram has a lowest discernible value of 1000 ns
+ * (Histogram::Options { 1000 }): its smallest bucket spans 0..511, so an empty
+ * histogram answers 511 for every percentile. Recorded deltas are at least the
+ * resolution (>= 1 ms), where that layout and this one share bucket bounds. */
+#define SCR_ELD_EMPTY_PERCENTILE 511
+
 /* hdr_value_at_percentile; the compiler checked Node's 0 < p <= 100 range. */
 double scr_eld_percentile(double handle, double percentile) {
   ScrEld *h = scr_eld_of(handle);
-  if (h == NULL || h->total == 0) return 0;
+  if (h == NULL || h->total == 0) return SCR_ELD_EMPTY_PERCENTILE;
   double requested = percentile < 100.0 ? percentile : 100.0;
   int64_t wanted = (int64_t)((requested / 100.0) * (double)h->total + 0.5);
   if (wanted < 1) wanted = 1;

tests/corpus/6397-event-loop-metrics.ts

@@ -15,6 +15,10 @@ let ticks = 0;
 // until the third sample.
 const keepAlive = setTimeout(() => console.log("never"), 10_000);
 const interval = setInterval(() => {
+  // A delta needs two samples after a reset. A loop that wakes late can run
+  // only one sample per window; that window is skipped without a reset, so
+  // the next one records.
+  if (histogram.count === 0) return;
   const now = performance.eventLoopUtilization();
   const delta = performance.eventLoopUtilization(now, last);
   last = now;
@@ -33,7 +37,14 @@ const interval = setInterval(() => {
     histogram.count > 0,
   );
   histogram.reset();
-  console.log("after reset", histogram.count, histogram.max, histogram.min, Number.isNaN(histogram.mean));
+  console.log(
+    "after reset",
+    histogram.count,
+    histogram.max,
+    histogram.min,
+    Number.isNaN(histogram.mean),
+    histogram.percentile(50),
+  );
   if (ticks === 3) {
     clearInterval(interval);
     clearTimeout(keepAlive);