@@ -101,7 +101,7 @@ github.com/docker/containerd 595e75c212d19a81d2b808a518fe1afc1391dad5

 github.com/tonistiigi/fifo 1405643975692217d6720f8b54aeee1bf2cd5cf4

 # cluster

-github.com/docker/swarmkit f93948cb430facd540e3c65c606384a89b3ac40f

+github.com/docker/swarmkit 17756457ad6dc4d8a639a1f0b7a85d1b65a617bb

 github.com/golang/mock bd3c8e81be01eef76d4b503f5e687d2d1354d2d9

 github.com/gogo/protobuf v0.3

 github.com/cloudflare/cfssl 7fb22c8cba7ecaf98e4082d22d65800cf45e042a

@@ -1,7 +1,6 @@

 package scheduler

 import (

-	"container/list"

 	"time"

 	"github.com/docker/swarmkit/api"

@@ -30,7 +29,7 @@ type schedulingDecision struct {

 // Scheduler assigns tasks to nodes.

 type Scheduler struct {

 	store           *store.MemoryStore

-	unassignedTasks *list.List

+	unassignedTasks map[string]*api.Task

 	// preassignedTasks already have NodeID, need resource validation

 	preassignedTasks map[string]*api.Task

 	nodeSet          nodeSet

@@ -47,7 +46,7 @@ type Scheduler struct {

 func New(store *store.MemoryStore) *Scheduler {

 	return &Scheduler{

 		store:            store,

-		unassignedTasks:  list.New(),

+		unassignedTasks:  make(map[string]*api.Task),

 		preassignedTasks: make(map[string]*api.Task),

 		allTasks:         make(map[string]*api.Task),

 		stopChan:         make(chan struct{}),

@@ -191,7 +190,7 @@ func (s *Scheduler) Stop() {

 // enqueue queues a task for scheduling.

 func (s *Scheduler) enqueue(t *api.Task) {

-	s.unassignedTasks.PushBack(t)

+	s.unassignedTasks[t.ID] = t

 }

 func (s *Scheduler) createTask(ctx context.Context, t *api.Task) int {

@@ -333,15 +332,12 @@ func (s *Scheduler) processPreassignedTasks(ctx context.Context) {

 // tick attempts to schedule the queue.

 func (s *Scheduler) tick(ctx context.Context) {

 	tasksByCommonSpec := make(map[string]map[string]*api.Task)

-	schedulingDecisions := make(map[string]schedulingDecision, s.unassignedTasks.Len())

+	schedulingDecisions := make(map[string]schedulingDecision, len(s.unassignedTasks))

-	var next *list.Element

-	for e := s.unassignedTasks.Front(); e != nil; e = next {

-		next = e.Next()

-		t := s.allTasks[e.Value.(*api.Task).ID]

+	for taskID, t := range s.unassignedTasks {

 		if t == nil || t.NodeID != "" {

 			// task deleted or already assigned

-			s.unassignedTasks.Remove(e)

+			delete(s.unassignedTasks, taskID)

 			continue

 		}

@@ -362,8 +358,8 @@ func (s *Scheduler) tick(ctx context.Context) {

 		if tasksByCommonSpec[taskGroupKey] == nil {

 			tasksByCommonSpec[taskGroupKey] = make(map[string]*api.Task)

 		}

-		tasksByCommonSpec[taskGroupKey][t.ID] = t

-		s.unassignedTasks.Remove(e)

+		tasksByCommonSpec[taskGroupKey][taskID] = t

+		delete(s.unassignedTasks, taskID)

 	}

 	for _, taskGroup := range tasksByCommonSpec {

@@ -540,6 +536,12 @@ func (s *Scheduler) scheduleTaskGroup(ctx context.Context, taskGroup map[string]

 	failedConstraints := make(map[int]bool) // key is index in nodes slice

 	nodeIter := 0

 	for taskID, t := range taskGroup {

+		// Skip tasks which were already scheduled because they ended

+		// up in two groups at once.

+		if _, exists := schedulingDecisions[taskID]; exists {

+			continue

+		}

+

 		n := &nodes[nodeIter%len(nodes)]

 		log.G(ctx).WithField("task.id", t.ID).Debugf("assigning to node %s", n.ID)