[#12] mclock: Fix timer-based scheduling

Let's assume that there are two requests in the queue with execution time t1 and t2.
The timer is set to t1. The timer is triggered, schedules the t1 request,
calculates the time for the next timer t2 to be triggered.
But it doesn't schedules timer to this time because of the
`q.timeBasedScheduleTs > nextTs` check.

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

scheduling/clock.go

@@ -1,6 +1,7 @@
 package scheduling
 
 import (
+	"math"
 	"sync"
 	"time"
 )
@@ -25,7 +26,7 @@ type systemClock struct {
 func newSystemClock() *systemClock {
 	c := &systemClock{
 		since:    time.Now(),
-		schedule: make(chan scheduleInfo),
+		schedule: make(chan scheduleInfo, 1),
 	}
 	c.start()
 	return c
@@ -36,10 +37,7 @@ func (c *systemClock) now() float64 {
 }
 
 func (c *systemClock) runAt(ts float64, f func()) {
-	select {
+	c.schedule <- scheduleInfo{ts: ts, f: f}
-	case c.schedule <- scheduleInfo{ts: ts, f: f}:
-	default: // timer fired, scheduleRequest will call runAt again
-	}
 }
 
 func (c *systemClock) close() {
@@ -53,6 +51,7 @@ func (c *systemClock) start() {
 		defer c.wg.Done()
 		t := time.NewTimer(0)
 		<-t.C
+		currentTs := math.MaxFloat64
 		var f func()
 		for {
 			select {
@@ -61,10 +60,16 @@ func (c *systemClock) start() {
 					f()
 					f = nil
 				}
+				currentTs = math.MaxFloat64
 			case s, ok := <-c.schedule:
 				if !ok {
 					return
 				}
+				if s.ts >= currentTs {
+					// current timer will fire earlier
+					// so next scheduleRequest will push new schedule event
+					continue
+				}
 				var d time.Duration
 				now := c.now()
 				if now < s.ts {
@@ -78,6 +83,7 @@ func (c *systemClock) start() {
 				}
 				t.Reset(d)
 				f = s.f
+				currentTs = s.ts
 			}
 		}
 	}()

scheduling/mclock.go

@@ -64,7 +64,6 @@ type MClock struct {
 	mtx              sync.Mutex
 	previous         map[string]*request
 	inProgress       uint64
-	timeBasedScheduleTs float64
 	reservationQueue *queue
 	limitQueue       *queue
 	sharesQueue      *queue
@@ -96,7 +95,6 @@ func NewMClock(runLimit, waitLimit uint64, tagInfo map[string]TagInfo, idleTimeo
 		limitQueue:       &queue{},
 		sharesQueue:      &queue{},
 		readyQueue:       &queue{},
-		timeBasedScheduleTs: math.MaxFloat64,
 	}
 
 	previous := make(map[string]*request)
@@ -294,13 +292,10 @@ func (q *MClock) setNextScheduleTimer(now float64) {
 		// should not happen as we always compare .ts() <= now
 		return
 	}
-
+	if nextTs == math.MaxFloat64 {
-	if q.timeBasedScheduleTs > nextTs {
+		return
-		q.clock.runAt(nextTs, func() {
-			q.scheduleRequest()
-		})
-		q.timeBasedScheduleTs = nextTs
 	}
+	q.clock.runAt(nextTs, q.scheduleRequest)
 }
 
 func (q *MClock) scheduleByLimitAndWeight(now float64) {

scheduling/mclock_test.go

@@ -493,3 +493,25 @@ func TestMClockTimeBasedSchedule(t *testing.T) {
 	close(checked)
 	require.NoError(t, eg.Wait())
 }
+
+func TestMClockLowLimit(t *testing.T) {
+	t.Parallel()
+	limit := 2.0
+	q, err := NewMClock(100, 100, map[string]TagInfo{
+		"class1": {Share: 50, LimitIOPS: &limit},
+	}, 5*time.Second)
+	require.NoError(t, err)
+	defer q.Close()
+
+	eg, ctx := errgroup.WithContext(context.Background())
+	eg.SetLimit(5)
+	eg.Go(func() error {
+		for range 3 {
+			release, err := q.RequestArrival(ctx, "class1")
+			require.NoError(t, err)
+			release()
+		}
+		return nil
+	})
+	require.NoError(t, eg.Wait())
+}