From c64048d73e5d0bc6ffc321b4ec13bce0f6098d47 Mon Sep 17 00:00:00 2001
From: Andrew M Bursavich <bursavich@google.com>
Date: Sat, 31 Oct 2015 22:33:02 -0700
Subject: [PATCH] Avoid full sort when selecting host with highest priority.

---
 plugin/pkg/scheduler/generic_scheduler.go | 39 +++++++++++------------
 1 file changed, 19 insertions(+), 20 deletions(-)

diff --git a/plugin/pkg/scheduler/generic_scheduler.go b/plugin/pkg/scheduler/generic_scheduler.go
index 649d89484a9..96ef1aef112 100644
--- a/plugin/pkg/scheduler/generic_scheduler.go
+++ b/plugin/pkg/scheduler/generic_scheduler.go
@@ -19,7 +19,6 @@ package scheduler
 import (
 	"fmt"
 	"math/rand"
-	"sort"
 	"strings"
 	"sync"
 
@@ -88,20 +87,32 @@ func (g *genericScheduler) Schedule(pod *api.Pod, nodeLister algorithm.NodeListe
 	return g.selectHost(priorityList)
 }
 
-// This method takes a prioritized list of nodes and sorts them in reverse order based on scores
-// and then picks one randomly from the nodes that had the highest score
+// This method takes a prioritized list of nodes, shuffles those with the highest scores to the
+// front of the list, and then picks one randomly from the nodes that had the highest score.
 func (g *genericScheduler) selectHost(priorityList algorithm.HostPriorityList) (string, error) {
 	if len(priorityList) == 0 {
 		return "", fmt.Errorf("empty priorityList")
 	}
-	sort.Sort(sort.Reverse(priorityList))
+	max := priorityList[0].Score
+	n := 0
+	for i, hostEntry := range priorityList {
+		if hostEntry.Score < max {
+			continue
+		}
+		if hostEntry.Score > max {
+			max = hostEntry.Score
+			n = 0
+		}
+		priorityList[i], priorityList[n] = priorityList[n], priorityList[i]
+		n++
+	}
+	if n == 1 {
+		return priorityList[0].Host, nil
+	}
 
-	hosts := getBestHosts(priorityList)
 	g.randomLock.Lock()
 	defer g.randomLock.Unlock()
-
-	ix := g.random.Int() % len(hosts)
-	return hosts[ix], nil
+	return priorityList[g.random.Intn(n)].Host, nil
 }
 
 // Filters the nodes to find the ones that fit based on the given predicate functions
@@ -179,18 +190,6 @@ func PrioritizeNodes(pod *api.Pod, podLister algorithm.PodLister, priorityConfig
 	return result, nil
 }
 
-func getBestHosts(list algorithm.HostPriorityList) []string {
-	result := []string{}
-	for _, hostEntry := range list {
-		if hostEntry.Score == list[0].Score {
-			result = append(result, hostEntry.Host)
-		} else {
-			break
-		}
-	}
-	return result
-}
-
 // EqualPriority is a prioritizer function that gives an equal weight of one to all nodes
 func EqualPriority(_ *api.Pod, podLister algorithm.PodLister, nodeLister algorithm.NodeLister) (algorithm.HostPriorityList, error) {
 	nodes, err := nodeLister.List()