diff --git a/src/evaluator.go b/src/evaluator.go
index 065229c..4996188 100644
--- a/src/evaluator.go
+++ b/src/evaluator.go
@@ -5,17 +5,15 @@ type Evaluator struct {
 	racks       map[string]map[string]int
 	nodes       map[string]map[string]int
 	upstreams   map[string]string
-	cost        float64
 	totalPS     int
 	totalWorker int
 
 	costNetwork float64
+	costLoad    float64
 
 	factorNode   float64
 	factorRack   float64
 	factorDomain float64
-
-	costLoad float64
 }
 
 func (eva *Evaluator) init(nodes []NodeStatus, tasks []Task) {
@@ -28,7 +26,6 @@ func (eva *Evaluator) init(nodes []NodeStatus, tasks []Task) {
 	eva.factorNode = 1.0
 	eva.factorRack = 4.0
 	eva.factorDomain = 40.0
-	eva.cost = 0.0
 	eva.costNetwork = 0.0
 	eva.costLoad = 0.0
 }
@@ -65,7 +62,6 @@ func (eva *Evaluator) add(node NodeStatus, task Task) {
 		eva.domains[node.Domain]["Worker"]++
 		eva.totalWorker++
 	}
-	eva.cost = eva.costNetwork
 
 	if task.IsPS {
 		//eva.costLoad += 1
@@ -104,7 +100,6 @@ func (eva *Evaluator) remove(node NodeStatus, task Task) {
 		eva.domains[node.Domain]["Worker"]--
 		eva.totalWorker--
 	}
-	eva.cost = eva.costNetwork
 
 	if task.IsPS {
 		//eva.costLoad -= 1
@@ -121,7 +116,10 @@ func (eva *Evaluator) remove(node NodeStatus, task Task) {
 }
 
 func (eva *Evaluator) calculate() float64 {
-	return eva.cost + eva.costLoad/float64(eva.totalPS+eva.totalWorker)
+	/* factor to determine spread or pack */
+	/* 1.0 spread, -1.0 pack */
+	factor := -1.0
+	return eva.costNetwork + factor*eva.costLoad/float64(eva.totalPS+eva.totalWorker)
 }
 
 func evaluate(allocation Allocation) float64 {
diff --git a/src/resource_pool.go b/src/resource_pool.go
index 30afec0..9da08fd 100644
--- a/src/resource_pool.go
+++ b/src/resource_pool.go
@@ -709,7 +709,8 @@ func (pool *ResourcePool) acquireResource(job Job) []NodeStatus {
 			for _, node := range cur.Nodes {
 				var available []GPUStatus
 				for _, status := range node.Status {
-					if status.MemoryAllocated == 0 && status.MemoryUsed < 10 {
+					/* make sure GPU is not used by in-system and outer-system */
+					if status.MemoryAllocated == 0 && status.MemoryUsed < 100 {
 						available = append(available, status)
 					}
 				}
@@ -720,7 +721,6 @@ func (pool *ResourcePool) acquireResource(job Job) []NodeStatus {
 					}
 				}
 			}
-			log.Info(candidates, cur)
 			if len(candidates) >= len(job.Tasks)*3+5 {
 				break
 			}
@@ -795,7 +795,33 @@ func (pool *ResourcePool) acquireResource(job Job) []NodeStatus {
 			nodesT = append(nodesT, node.Copy())
 		}
 
-		allocation := fastBestFit(nodesT, job.Tasks)
+		tasks := make([]Task, len(job.Tasks))
+		var tasksPS []Task
+		var tasksWorker []Task
+		for _, taskT := range job.Tasks {
+			if taskT.IsPS {
+				tasksPS = append(tasksPS, taskT)
+			} else {
+				tasksWorker = append(tasksWorker, taskT)
+			}
+		}
+		idxPS := 0
+		idxWorker := 0
+		factor := float64(len(tasksWorker)) / (float64(len(tasksPS)) + 0.001)
+		for i := range tasks {
+			if float64(idxPS)*factor <= float64(idxWorker) && idxPS < len(tasksPS) {
+				tasks[i] = tasksPS[idxPS]
+				idxPS++
+			} else if idxWorker < len(tasksWorker) {
+				tasks[i] = tasksWorker[idxWorker]
+				idxWorker++
+			} else {
+				tasks[i] = tasksPS[idxPS]
+				idxPS++
+			}
+		}
+
+		allocation := fastBestFit(nodesT, tasks)
 		if allocation.Flags["valid"] {
 
 			for range job.Tasks { //append would cause uncertain order