p2p/msgrate: be more lenient when calculating 'mean' (ethereum#25653)

The p2p msgrate tracker is a thing which tries to estimate some mean round-trip times. However, it did so in a very curious way: if a node had 200 peers, it would sort their 200 respective rtt estimates, and then it would pick item number 2 as the mean. So effectively taking third fastest and calling it mean. This probably works "ok" when the number of peers are low (there are other factors too, such as ttlScaling which takes some of the edge off this) -- however when the number of peers is high, it becomes very skewed.

This PR instead bases the 'mean' on the square root of the length of the list. Still pretty harsh, but a bit more lenient.

p2p/msgrate/msgrate.go

@@ -38,14 +38,6 @@ const measurementImpact = 0.1
 // to fetch more than some local stable value.
 const capacityOverestimation = 1.01
 
-// qosTuningPeers is the number of best peers to tune round trip times based on.
-// An Ethereum node doesn't need hundreds of connections to operate correctly,
-// so instead of lowering our download speed to the median of potentially many
-// bad nodes, we can target a smaller set of vey good nodes. At worse this will
-// result in less nodes to sync from, but that's still better than some hogging
-// the pipeline.
-const qosTuningPeers = 5
-
 // rttMinEstimate is the minimal round trip time to target requests for. Since
 // every request entails a 2 way latency + bandwidth + serving database lookups,
 // it should be generous enough to permit meaningful work to be done on top of
@@ -303,11 +295,15 @@ func (t *Trackers) medianRoundTrip() time.Duration {
 	}
 	sort.Float64s(rtts)
 
-	median := rttMaxEstimate
-	if qosTuningPeers <= len(rtts) {
-		median = time.Duration(rtts[qosTuningPeers/2]) // Median of our best few peers
-	} else if len(rtts) > 0 {
-		median = time.Duration(rtts[len(rtts)/2]) // Median of all out connected peers
+	var median time.Duration
+	switch len(rtts) {
+	case 0:
+		median = rttMaxEstimate
+	case 1:
+		median = time.Duration(rtts[0])
+	default:
+		idx := int(math.Sqrt(float64(len(rtts))))
+		median = time.Duration(rtts[idx])
 	}
 	// Restrict the RTT into some QoS defaults, irrelevant of true RTT
 	if median < rttMinEstimate {