Take the cable zero from the shortest delay seen rather than a flag

main.go

@@ -539,7 +539,6 @@ type config struct {
 	streams    int
 	batch      int
 	probeEther uint16
-	zeroNS     float64
 	nsPerM     float64
 }
 
@@ -550,13 +549,13 @@ func main() {
 		sizesArg = flag.String("sizes", "64,128,256,512,1024,1280,1514", "frame sizes in bytes, excluding FCS, cycled per packet")
 		streams  = flag.Int("streams", 7, "independent streams per direction, capped by rx rings; each gets its own ethertype, steered by a flow rule to its own rx queue")
 		batch    = flag.Int("batch", 64, "frames per sendmmsg/recvmmsg call")
-		zeroNS   = flag.Float64("zero-ns", 2163.77, "mean of both directions at zero cable length; belongs to the media adapters, recalibrate when they change")
+
-		nsPerM   = flag.Float64("ns-per-m", 5.4545, "mean of both directions, per metre of cable")
+		nsPerM = flag.Float64("ns-per-m", 5.3, "mean of both directions, per metre of cable")
 	)
 	flag.Parse()
 
 	if err := run(*aName, *bName, *sizesArg,
-		*streams, *batch, *zeroNS, *nsPerM); err != nil {
+		*streams, *batch, *nsPerM); err != nil {
 		fmt.Fprintln(os.Stderr, "error:", err)
 		os.Exit(1)
 	}
@@ -574,7 +573,7 @@ const (
 )
 
 func run(aName, bName, sizesArg string,
-	nStreams, batch int, zeroNS, nsPerM float64) error {
+	nStreams, batch int, nsPerM float64) error {
 
 	if aName == "" || bName == "" {
 		return fmt.Errorf("both -a and -b are required")
@@ -629,7 +628,6 @@ func run(aName, bName, sizesArg string,
 		streams:    nStreams,
 		batch:      batch,
 		probeEther: uint16(etherBase + nStreams),
-		zeroNS:     zeroNS,
 		nsPerM:     nsPerM,
 	}
 
@@ -674,7 +672,7 @@ func run(aName, bName, sizesArg string,
 				nStreams, ethertypes[0], ethertypes[len(ethertypes)-1])},
 			{"probe", fmt.Sprintf("ethertype 0x%04x every %s", cfg.probeEther, probeInterval)},
 			{"batch", fmt.Sprintf("%d frames per syscall", batch)},
-			{"calibration", fmt.Sprintf("%g ns at zero length, %g ns/m", zeroNS, nsPerM)},
+			{"calibration", fmt.Sprintf("%g ns/m, zero taken from the shortest delay seen so far", nsPerM)},
 			{"buffers", fmt.Sprintf("sndbuf %s, rcvbuf %s",
 				humanBytes(uint64(sockBufSize(dirs[0].txFDs[0], unix.SO_SNDBUF))),
 				humanBytes(uint64(sockBufSize(dirs[0].rxFDs[0], unix.SO_RCVBUF))))},

probe.go

@@ -34,18 +34,20 @@ type cableStats struct {
 	mu      sync.Mutex
 	min     int64
 	samples uint64
+	floor   int64
 
 	txPend map[uint64]int64
 	rxPend map[uint64]int64
 }
 
 type cableView struct {
-	min     int64
+	min   int64
-	samples uint64
+	floor int64
+	ok    bool
 }
 
 func (v cableView) minText() string {
-	if v.samples == 0 {
+	if !v.ok {
 		return "-"
 	}
 	return commasInt(v.min)
@@ -57,15 +59,14 @@ func (c config) cableMetres(views []view) (float64, bool) {
 	if len(views) == 0 {
 		return 0, false
 	}
-	var sum float64
+	var excess float64
 	for _, v := range views {
-		if v.cable.samples == 0 {
+		if !v.cable.ok {
 			return 0, false
 		}
-		sum += float64(v.cable.min)
+		excess += float64(v.cable.min - v.cable.floor)
 	}
-	mean := sum / float64(len(views))
+	return excess / float64(len(views)) / c.nsPerM, true
-	return (mean - c.zeroNS) / c.nsPerM, true
 }
 
 func (c config) cableText(views []view) string {
@@ -114,13 +115,16 @@ func (c *cableStats) put(seq uint64, ts int64, tx bool) {
 	if c.samples == 0 || delta < c.min {
 		c.min = delta
 	}
+	if c.floor == 0 || delta < c.floor {
+		c.floor = delta
+	}
 	c.samples++
 }
 
 func (c *cableStats) view() cableView {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	return cableView{c.min, c.samples}
+	return cableView{c.min, c.floor, c.samples > 0}
 }
 
 func (c *cableStats) reset() {