govarnishlog can print a r/w channel length high watermark.

cmd/govarnishlog/govarnishlog.go

@@ -67,6 +67,7 @@ var (
 	eol          = uint64(0)
 	printTxFunc  = printTxTerse
 	printRecFunc = printRecTerse
+	chanHi       = 0
 )
 
 func newIdle(seen uint64, seenLast uint64, t time.Time,
@@ -112,6 +113,10 @@ func txReader(q *log.Query, txChan chan []log.Tx, statusChan chan log.Status,
 			return
 		}
 		txChan <- txGrp
+		chanLen := len(txChan)
+		if chanLen > chanHi {
+			chanHi = chanLen
+		}
 		seen++
 	}
 }
@@ -252,6 +257,8 @@ LOOP:
 			stop.Sub(start))
 		fmt.Fprintf(os.Stderr, "%d tx grps read\n", seen)
 		fmt.Fprintf(os.Stderr, "%d tx grps written\n", n)
+		fmt.Fprintf(os.Stderr, "%d r/w channel high watermark\n",
+			chanHi)
 		fmt.Fprintf(os.Stderr, "%d times reader at eol\n", eol)
 		fmt.Fprintf(os.Stderr, "%v reader total idle time\n", totIdle)
 		fmt.Fprintf(os.Stderr, "%v mean idle time\n",