Increase transmitter concurrency

MERC-6635

core/services/llo/mercurytransmitter/transmitter.go

@@ -29,6 +29,13 @@ import (
 const (
 	// Mercury server error codes
 	DuplicateReport = 2
+
+	// TransmitConcurrency is the number of concurrent transmits to each
+	// server. Number of goroutines per server will be roughly 2x this because
+	// each server has a delete queue and a transmit queue.
+	//
+	// This could be reduced by implementing transmit batching, see: https://smartcontract-it.atlassian.net/browse/MERC-6635
+	TransmitConcurrency = 100
 )
 
 var (
@@ -167,9 +174,12 @@ func (mt *transmitter) Start(ctx context.Context) (err error) {
 				// starting pm after loading from it is fine because it simply spawns some garbage collection/prune goroutines
 				startClosers = append(startClosers, s.c, s.q, s.pm)
 
-				mt.wg.Add(2)
-				go s.runDeleteQueueLoop(mt.stopCh, mt.wg)
-				go s.runQueueLoop(mt.stopCh, mt.wg, fmt.Sprintf("%d", mt.donID))
+				nThreads := int(TransmitConcurrency)
+				mt.wg.Add(2 * nThreads)
+				for i := 0; i < nThreads; i++ {
+					go s.runDeleteQueueLoop(mt.stopCh, mt.wg)
+					go s.runQueueLoop(mt.stopCh, mt.wg, fmt.Sprintf("%d", mt.donID))
+				}
 			}
 			if err := (&services.MultiStart{}).Start(ctx, startClosers...); err != nil {
 				return err