fix: harden single-machine load handling

loadtest/im-pressure/agent.go

@@ -458,8 +458,18 @@ func newHistoryQuery(client *WSClient, topic string, messages []AcceptedMessage,
 	for _, message := range messages {
 		expected = append(expected, message.ClientMid)
 	}
-	return historyQuery{client: client, topic: topic, since: messages[0].Seq,
-		before: messages[len(messages)-1].Seq + 1, limit: limit, expected: expected}
+	since := messages[0].Seq
+	before := messages[len(messages)-1].Seq + 1
+	// P2P topics interleave both senders. The requested range must leave room
+	// for non-target messages between the expected sequence numbers.
+	if span := before - since; span > int64(limit) {
+		limit = int(span)
+	}
+	if limit > 200 {
+		limit = 200
+	}
+	return historyQuery{client: client, topic: topic, since: since,
+		before: before, limit: limit, expected: expected}
 }
 
 func (r *agentRun) waitHistoryPage(ctx context.Context, expected []string, timeout time.Duration) {
@@ -659,13 +669,24 @@ func (r *agentRun) execute(ctx context.Context, start time.Time, groupTopic stri
 	case "p2p":
 		return r.scheduleP2P(ctx, start, r.cfg.UsersPerMachine, r.cfg.P2PMessages, mustDuration(r.cfg.P2PInterval))
 	case "group":
-		return r.scheduleGroup(ctx, start, groupTopic, 1, 500, 2)
+		return r.scheduleGroup(ctx, start, groupTopic, 1, scaledUsers(r.cfg.UsersPerMachine, 1, 2), 2)
 	case "mixed":
+		p2pUsers := scaledUsers(r.cfg.UsersPerMachine, 3, 10)
+		groupSenders := scaledUsers(r.cfg.UsersPerMachine, 1, 4)
+		if p2pUsers+groupSenders > r.cfg.UsersPerMachine {
+			groupSenders = r.cfg.UsersPerMachine - p2pUsers
+		}
+		if groupSenders <= 0 {
+			return fmt.Errorf("users_per_machine=%d is too small for mixed scenario", r.cfg.UsersPerMachine)
+		}
 		var wg sync.WaitGroup
 		errCh := make(chan error, 2)
 		wg.Add(2)
-		go func() { defer wg.Done(); errCh <- r.scheduleP2P(ctx, start, 300, 5, 20*time.Second) }()
-		go func() { defer wg.Done(); errCh <- r.scheduleGroup(ctx, start, groupTopic, 301, 550, 2) }()
+		go func() { defer wg.Done(); errCh <- r.scheduleP2P(ctx, start, p2pUsers, 5, 20*time.Second) }()
+		go func() {
+			defer wg.Done()
+			errCh <- r.scheduleGroup(ctx, start, groupTopic, p2pUsers+1, p2pUsers+groupSenders, 2)
+		}()
 		wg.Wait()
 		close(errCh)
 		for err := range errCh {
@@ -679,6 +700,17 @@ func (r *agentRun) execute(ctx context.Context, start time.Time, groupTopic stri
 	}
 }
 
+func scaledUsers(total, numerator, denominator int) int {
+	count := total * numerator / denominator
+	if count < 1 {
+		return 1
+	}
+	if count > total {
+		return total
+	}
+	return count
+}
+
 func (r *agentRun) scheduleP2P(ctx context.Context, start time.Time, users, messages int, interval time.Duration) error {
 	var wg sync.WaitGroup
 	for index := 1; index <= users; index++ {
@@ -858,9 +890,10 @@ func (r *agentRun) drainDuration() (time.Duration, error) {
 
 func (r *agentRun) close() {
 	clients := r.clientsSnapshot()
-	var interval time.Duration
-	if r.cfg != nil && r.cfg.CloseRamp != "" && len(clients) > 1 {
-		interval = mustDuration(r.cfg.CloseRamp) / time.Duration(len(clients)-1)
+	sort.Slice(clients, func(i, j int) bool { return clients[i].account.Index < clients[j].account.Index })
+	interval, initialDelay := closeSchedule(r.cfg, r.machine, len(clients))
+	if initialDelay > 0 {
+		time.Sleep(initialDelay)
 	}
 	for index, client := range clients {
 		client.Close()
@@ -870,6 +903,19 @@ func (r *agentRun) close() {
 	}
 }
 
+func closeSchedule(cfg *Config, machine string, clients int) (interval, initialDelay time.Duration) {
+	if cfg == nil || cfg.CloseRamp == "" || clients <= 1 {
+		return 0, 0
+	}
+	// The two logical agents share one global ramp. Interleaving their slots avoids
+	// doubling the disconnect rate when both agents begin cleanup at the same time.
+	interval = 2 * mustDuration(cfg.CloseRamp) / time.Duration(clients-1)
+	if machine == "b" {
+		initialDelay = interval / 2
+	}
+	return interval, initialDelay
+}
+
 func waitUntil(ctx context.Context, target time.Time) error {
 	delay := time.Until(target)
 	if delay <= 0 {