[#9999] mclock: Schedule by limit requests as soon as possible

Let's assume that for some tag `limit = 1000 RPS` defined and each
request takes 10 ms to complete. At some point in time 1000 requests
were accepted. Then first request will be scheduled at `now()`, second -
at `now() + 1 ms`, third - at `now() + 2 ms` etc. Total processing
duration of 1000 requests will be 1 second + 10 ms.

After this fix scheduler looks forward to schedule requests within limit.
So for situation above total processing duration of 1000 requests will be
10 ms in ideal world.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

scheduling/mclock.go

@@ -306,7 +306,7 @@ func (q *MClock) setNextScheduleTimer(now float64) {
 }
 
 func (q *MClock) scheduleByLimitAndWeight(now float64) {
-	for q.limitQueue.Len() > 0 && q.limitQueue.items[0].ts() <= now {
+	for q.limitQueue.Len() > 0 && q.limitQueue.items[0].ts() < now+1.0 {
 		ready := heap.Pop(q.limitQueue).(*limitMQueueItem)
 		heap.Push(q.readyQueue, &readyMQueueItem{r: ready.r})
 	}

scheduling/mclock_test.go

@@ -515,3 +515,48 @@ func TestMClockLowLimit(t *testing.T) {
 	})
 	require.NoError(t, eg.Wait())
 }
+
+func TestMClockLimitTotalTime(t *testing.T) {
+	t.Parallel()
+	limit := 10.0 // 10 RPS -> 1 request per 100 ms
+	q, err := NewMClock(100, 100, map[string]TagInfo{
+		"class1": {Share: 50, LimitIOPS: &limit},
+	}, 5*time.Second)
+	require.NoError(t, err)
+	defer q.Close()
+
+	// 10 requests, each request runs for 500 ms,
+	// but they should be scheduled as soon as possible,
+	// so total duration must be less than 1 second
+	eg, ctx := errgroup.WithContext(context.Background())
+	startedAt := time.Now()
+	for range 10 {
+		eg.Go(func() error {
+			release, err := q.RequestArrival(ctx, "class1")
+			require.NoError(t, err)
+			time.Sleep(500 * time.Millisecond)
+			release()
+			return nil
+		})
+	}
+	require.NoError(t, eg.Wait())
+	require.True(t, time.Since(startedAt) <= 1*time.Second)
+
+	// 11 requests, limit = 10 RPS, so 10 requests should be
+	// scheduled as soon as possible, but last request should be
+	// scheduled at now + 1.0 s
+	eg, ctx = errgroup.WithContext(context.Background())
+	startedAt = time.Now()
+	for range 11 {
+		eg.Go(func() error {
+			release, err := q.RequestArrival(ctx, "class1")
+			require.NoError(t, err)
+			time.Sleep(500 * time.Millisecond)
+			release()
+			return nil
+		})
+	}
+	require.NoError(t, eg.Wait())
+	require.True(t, time.Since(startedAt) >= 1500*time.Millisecond)
+	require.True(t, time.Since(startedAt) <= 1600*time.Millisecond) // 100 ms offset to complete all requests
+}