接着上篇来说。hadoop首先调度辅助型task(job-cleanup task、task-cleanup task和job-setup task)，这是由JobTracker来完成的；但对于计算型task，则是由作业调度器TaskScheduler来分配的，其默认实现为JobQueueTaskScheduler。具体过程在assignTasks()方法中完成，下面来一段一段的分析该方法。
public synchronized List<Task> assignTasks(TaskTracker taskTracker)
throws IOException {
// Check for JT safe-mode
if (taskTrackerManager.isInSafeMode()) {
LOG.info("JobTracker is in safe-mode, not scheduling any tasks.");
return null;
}
TaskTrackerStatus taskTrackerStatus = taskTracker.getStatus();
ClusterStatus clusterStatus = taskTrackerManager.getClusterStatus();
final int numTaskTrackers = clusterStatus.getTaskTrackers();
final int clusterMapCapacity = clusterStatus.getMaxMapTasks();
final int clusterReduceCapacity = clusterStatus.getMaxReduceTasks();
Collection<JobInProgress> jobQueue =
jobQueueJobInProgressListener.getJobQueue();
首先检查是否处于安全模式；接着分别获取该TaskTracker的状态信息、集群状态信息、集群中的TaskTracker数目、集群能运行的最大Map Task个数和Reduce Task个数；再选择一个作业队列，对该队列中的作业进行调度。
1 //
2 // Get map + reduce counts for the current tracker.
3 //
4 final int trackerMapCapacity = taskTrackerStatus.getMaxMapSlots();
5 final int trackerReduceCapacity = taskTrackerStatus.getMaxReduceSlots();
6 final int trackerRunningMaps = taskTrackerStatus.countMapTasks();
7 final int trackerRunningReduces = taskTrackerStatus.countReduceTasks();
8
9 // Assigned tasks
10 List<Task> assignedTasks = new ArrayList<Task>();
这4行分别是获取Map和Reduce的slot，然后是获取当前TaskTracker上正在运行的Map和Reduce task数目；最后一行的集合用来存放分配给该TaskTracker的task。
1 //
2 // Compute (running + pending) map and reduce task numbers across pool
3 //
4 int remainingReduceLoad = 0;
5 int remainingMapLoad = 0;
6 synchronized (jobQueue) {
7 for (JobInProgress job : jobQueue) {
8 if (job.getStatus().getRunState() == JobStatus.RUNNING) {
9 remainingMapLoad += (job.desiredMaps() - job.finishedMaps());
10 if (job.scheduleReduces()) {
11 remainingReduceLoad +=
12 (job.desiredReduces() - job.finishedReduces());
13 }
14 }
15 }
16 }
该段代码用来计算作业队列中还有多少Map和Reduce task需要运行。job.desiredMaps()方法用来计算该Job总共有多少个Map task。job.finishedMaps()方法用来计算该Job有多少个已完成的Map task。同理，job.desiredReduces()方法与job.finishedReduces()方法用来计算Reduce。
// Compute the 'load factor' for maps and reduces
double mapLoadFactor = 0.0;
if (clusterMapCapacity > 0) {
mapLoadFactor = (double)remainingMapLoad / clusterMapCapacity;
}
double reduceLoadFactor = 0.0;
if (clusterReduceCapacity > 0) {
reduceLoadFactor = (double)remainingReduceLoad / clusterReduceCapacity;
}
用来计算Map和Reduce task的装载百分比，即根据剩余需要运行的Map task和集群能运行的最大Map Task个数的比例，来为TaskTracker计算一个装载因子，使得该TaskTracker上的Map task个数不超过这个比例。Reduce也一样。
1 //
2 // In the below steps, we allocate first map tasks (if appropriate),
3 // and then reduce tasks if appropriate. We go through all jobs
4 // in order of job arrival; jobs only get serviced if their
5 // predecessors are serviced, too.
6 //
7
8 //
9 // We assign tasks to the current taskTracker if the given machine
10 // has a workload that's less than the maximum load of that kind of
11 // task.
12 // However, if the cluster is close to getting loaded i.e. we don't
13 // have enough _padding_ for speculative executions etc., we only
14 // schedule the "highest priority" task i.e. the task from the job
15 // with the highest priority.
16 //
17
18 final int trackerCurrentMapCapacity =
19 Math.min((int)Math.ceil(mapLoadFactor * trackerMapCapacity),
20 trackerMapCapacity);
21 int availableMapSlots = trackerCurrentMapCapacity - trackerRunningMaps;
22 boolean exceededMapPadding = false;
23 if (availableMapSlots > 0) {
24 exceededMapPadding =
25 exceededPadding(true, clusterStatus, trackerMapCapacity);
26 }
第一行根据上一步计算出来的Map task装载因子，计算当前结点能够运行的Map task个数；第二行计算剩余的能够运行Map task的slot个数availableMapSlots。如果availableMapSlots大于0表示还有余地运行Map task。Hadoop不会把所有的slot 都分配完，而是会留一些slot给失败的和推测执行的任务，exceededPadding()方法就是来完成这个任务的。
1 int numLocalMaps = 0;
2 int numNonLocalMaps = 0;
3 scheduleMaps:
4 for (int i=0; i < availableMapSlots; ++i) {
5 synchronized (jobQueue) {
6 for (JobInProgress job : jobQueue) {
7 if (job.getStatus().getRunState() != JobStatus.RUNNING) {
8 continue;
9 }
10
11 Task t = null;
12
13 // Try to schedule a Map task with locality between node-local
14 // and rack-local
15 t =
16 job.obtainNewNodeOrRackLocalMapTask(taskTrackerStatus,
17 numTaskTrackers, taskTrackerManager.getNumberOfUniqueHosts());
18 if (t != null) {
19 assignedTasks.add(t);
20 ++numLocalMaps;
21
22 // Don't assign map tasks to the hilt!
23 // Leave some free slots in the cluster for future task-failures,
24 // speculative tasks etc. beyond the highest priority job
25 if (exceededMapPadding) {
26 break scheduleMaps;
27 }
28
29 // Try all jobs again for the next Map task
30 break;
31 }
32
33 // Try to schedule a node-local or rack-local Map task
34 t =
35 job.obtainNewNonLocalMapTask(taskTrackerStatus, numTaskTrackers,
36 taskTrackerManager.getNumberOfUniqueHosts());
37
38 if (t != null) {
39 assignedTasks.add(t);
40 ++numNonLocalMaps;
41
42 // We assign at most 1 off-switch or speculative task
43 // This is to prevent TaskTrackers from stealing local-tasks
44 // from other TaskTrackers.
45 break scheduleMaps;
46 }
47 }
48 }
49 }
50 int assignedMaps = assignedTasks.size();
以上这部分就是分配Map task的过程。obtainNewNodeOrRackLocalMapTask()方法和obtainNewNonLocalMapTask()方法分别用来分配node-local/rack-local task和非本地的task(我觉得hadoop中这个方法的注释写的有问题，第33行，原代码第195行)。他们最终都调用了findNewMapTask()方法来分配task，但区别在于调用时的级别：obtainNewNodeOrRackLocalMapTask ()方法是“maxLevel”，表示可以运行node-local/rack-local级别的task，obtainNewNonLocalMapTask()方法是“NON_LOCAL_CACHE_LEVEL”，表示只能运行off-switch/speculative级别的task。而“anyCacheLevel”级别最高，表示node-local, rack-local, off-switch and speculative task都可以分配。
1 1 /** 2 2 * Find new map task 3 3 * @param tts The task tracker that is asking for a task 4 4 * @param clusterSize The number of task trackers in the cluster 5 5 * @param numUniqueHosts The number of hosts that run task trackers 6 6 * @param avgProgress The average progress of this kind of task in this job 7 7 * @param maxCacheLevel The maximum topology level until which to schedule 8 8 * maps. 9 9 * A value of {@link #anyCacheLevel} implies any 10 10 * available task (node-local, rack-local, off-switch and 11 11 * speculative tasks). 12 12 * A value of {@link #NON_LOCAL_CACHE_LEVEL} implies only 13 13 * off-switch/speculative tasks should be scheduled. 14 14 * @return the index in tasks of the selected task (or -1 for no task) 15 15 */ 16 16 private synchronized int findNewMapTask(final TaskTrackerStatus tts, 17 17 final int clusterSize, 18 18 final int numUniqueHosts, 19 19 final int maxCacheLevel, 20 20 final double avgProgress) { 21 21 if (numMapTasks == 0) { 22 22 if(LOG.isDebugEnabled()) { 23 23 LOG.debug("No maps to schedule for " + profile.getJobID()); 24 24 } 25 25 return -1; 26 26 } 27 27 28 28 String taskTracker = tts.getTrackerName(); 29 29 TaskInProgress tip = null; 30 30 31 31 // 32 32 // Update the last-known clusterSize 33 33 // 34 34 this.clusterSize = clusterSize; 35 35 36 36 if (!shouldRunOnTaskTracker(taskTracker)) { 37 37 return -1; 38 38 } 39 39 40 40 // Check to ensure this TaskTracker has enough resources to 41 41 // run tasks from this job 42 42 long outSize = resourceEstimator.getEstimatedMapOutputSize(); 43 43 long availSpace = tts.getResourceStatus().getAvailableSpace(); 44 44 if(availSpace < outSize) { 45 45 LOG.warn("No room for map task. Node " + tts.getHost() + 46 46 " has " + availSpace + 47 47 " bytes free; but we expect map to take " + outSize); 48 48 49 49 return -1; //see if a different TIP might work better. 50 50 } 51 51 52 52 53 53 // When scheduling a map task: 54 54 // 0) Schedule a failed task without considering locality 55 55 // 1) Schedule non-running tasks 56 56 // 2) Schedule speculative tasks 57 57 // 3) Schedule tasks with no location information 58 58 59 59 // First a look up is done on the non-running cache and on a miss, a look 60 60 // up is done on the running cache. The order for lookup within the cache: 61 61 // 1. from local node to root [bottom up] 62 62 // 2. breadth wise for all the parent nodes at max level 63 63 // We fall to linear scan of the list ((3) above) if we have misses in the 64 64 // above caches 65 65 66 66 // 0) Schedule the task with the most failures, unless failure was on this 67 67 // machine 68 68 tip = findTaskFromList(failedMaps, tts, numUniqueHosts, false); 69 69 if (tip != null) { 70 70 // Add to the running list 71 71 scheduleMap(tip); 72 72 LOG.info("Choosing a failed task " + tip.getTIPId()); 73 73 return tip.getIdWithinJob(); 74 74 } 75 75 76 76 Node node = jobtracker.getNode(tts.getHost()); 77 77 78 78 // 79 79 // 1) Non-running TIP : 80 80 // 81 81 82 82 // 1. check from local node to the root [bottom up cache lookup] 83 83 // i.e if the cache is available and the host has been resolved 84 84 // (node!=null) 85 85 if (node != null) { 86 86 Node key = node; 87 87 int level = 0; 88 88 // maxCacheLevel might be greater than this.maxLevel if findNewMapTask is 89 89 // called to schedule any task (local, rack-local, off-switch or 90 90 // speculative) tasks or it might be NON_LOCAL_CACHE_LEVEL (i.e. -1) if 91 91 // findNewMapTask is (i.e. -1) if findNewMapTask is to only schedule 92 92 // off-switch/speculative tasks 93 93 int maxLevelToSchedule = Math.min(maxCacheLevel, maxLevel); 94 94 for (level = 0;level < maxLevelToSchedule; ++level) { 95 95 List <TaskInProgress> cacheForLevel = nonRunningMapCache.get(key); 96 96 if (cacheForLevel != null) { 97 97 tip = findTaskFromList(cacheForLevel, tts, 98 98 numUniqueHosts,level == 0); 99 99 if (tip != null) { 100 100 // Add to running cache 101 101 scheduleMap(tip); 102 102 103 103 // remove the cache if its empty 104 104 if (cacheForLevel.size() == 0) { 105 105 nonRunningMapCache.remove(key); 106 106 } 107 107 108 108 return tip.getIdWithinJob(); 109 109 } 110 110 } 111 111 key = key.getParent(); 112 112 } 113 113 114 114 // Check if we need to only schedule a local task (node-local/rack-local) 115 115 if (level == maxCacheLevel) { 116 116 return -1; 117 117 } 118 118 } 119 119 120 120 //2. Search breadth-wise across parents at max level for non-running 121 121 // TIP if 122 122 // - cache exists and there is a cache miss 123 123 // - node information for the tracker is missing (tracker's topology 124 124 // info not obtained yet) 125 125 126 126 // collection of node at max level in the cache structure 127 127 Collection<Node> nodesAtMaxLevel = jobtracker.getNodesAtMaxLevel(); 128 128 129 129 // get the node parent at max level 130 130 Node nodeParentAtMaxLevel = 131 131 (node == null) ? null : JobTracker.getParentNode(node, maxLevel - 1); 132 132 133 133 for (Node parent : nodesAtMaxLevel) { 134 134 135 135 // skip the parent that has already been scanned 136 136 if (parent == nodeParentAtMaxLevel) { 137 137 continue; 138 138 } 139 139 140 140 List<TaskInProgress> cache = nonRunningMapCache.get(parent); 141 141 if (cache != null) { 142 142 tip = findTaskFromList(cache, tts, numUniqueHosts, false); 143 143 if (tip != null) { 144 144 // Add to the running cache 145 145 scheduleMap(tip); 146 146 147 147 // remove the cache if empty 148 148 if (cache.size() == 0) { 149 149 nonRunningMapCache.remove(parent); 150 150 } 151 151 LOG.info("Choosing a non-local task " + tip.getTIPId()); 152 152 return tip.getIdWithinJob(); 153 153 } 154 154 } 155 155 } 156 156 157 157 // 3. Search non-local tips for a new task 158 158 tip = findTaskFromList(nonLocalMaps, tts, numUniqueHosts, false); 159 159 if (tip != null) { 160 160 // Add to the running list 161 161 scheduleMap(tip); 162 162 163 163 LOG.info("Choosing a non-local task " + tip.getTIPId()); 164 164 return tip.getIdWithinJob(); 165 165 } 166 166 167 167 // 168 168 // 2) Running TIP : 169 169 // 170 170 171 171 if (hasSpeculativeMaps) { 172 172 long currentTime = jobtracker.getClock().getTime(); 173 173 174 174 // 1. Check bottom up for speculative tasks from the running cache 175 175 if (node != null) { 176 176 Node key = node; 177 177 for (int level = 0; level < maxLevel; ++level) { 178 178 Set<TaskInProgress> cacheForLevel = runningMapCache.get(key); 179 179 if (cacheForLevel != null) { 180 180 tip = findSpeculativeTask(cacheForLevel, tts, 181 181 avgProgress, currentTime, level == 0); 182 182 if (tip != null) { 183 183 if (cacheForLevel.size() == 0) { 184 184 runningMapCache.remove(key); 185 185 } 186 186 return tip.getIdWithinJob(); 187 187 } 188 188 } 189 189 key = key.getParent(); 190 190 } 191 191 } 192 192 193 193 // 2. Check breadth-wise for speculative tasks 194 194 195 195 for (Node parent : nodesAtMaxLevel) { 196 196 // ignore the parent which is already scanned 197 197 if (parent == nodeParentAtMaxLevel) { 198 198 continue; 199 199 } 200 200 201 201 Set<TaskInProgress> cache = runningMapCache.get(parent); 202 202 if (cache != null) { 203 203 tip = findSpeculativeTask(cache, tts, avgProgress, 204 204 currentTime, false); 205 205 if (tip != null) { 206 206 // remove empty cache entries 207 207 if (cache.size() == 0) { 208 208 runningMapCache.remove(parent); 209 209 } 210 210 LOG.info("Choosing a non-local task " + tip.getTIPId() 211 211 + " for speculation"); 212 212 return tip.getIdWithinJob(); 213 213 } 214 214 } 215 215 } 216 216 217 217 // 3. Check non-local tips for speculation 218 218 tip = findSpeculativeTask(nonLocalRunningMaps, tts, avgProgress, 219 219 currentTime, false); 220 220 if (tip != null) { 221 221 LOG.info("Choosing a non-local task " + tip.getTIPId() 222 222 + " for speculation"); 223 223 return tip.getIdWithinJob(); 224 224 } 225 225 } 226 226 227 227 return -1; 228 228 }
这里穿插说一下findNewMapTask()方法，真正的任务分配都是它来做的，task分配的优先级为：
1）、从failedMaps中调度failed Task
2）、从nonRunningMapCache中选择具有本地性的任务，优先级为node-local、rack-local、off-switch。至于本地性如何体现在后边说。
3）、从nonLocalMaps中选择任务
4）、从runningMapCache中选择任务，为其启动备份执行
5）、从nonLocalRunningMaps中选择任务，为其启动备份执行
最后，如果findNewMapTask()方法返回值为-1，则表示没有找到合适的Map task。否则返回值表示该Map task在JobInProgress的maps[]数组中的下标。
1 //
2 // Same thing, but for reduce tasks
3 // However we _never_ assign more than 1 reduce task per heartbeat
4 //
5 final int trackerCurrentReduceCapacity =
6 Math.min((int)Math.ceil(reduceLoadFactor * trackerReduceCapacity),
7 trackerReduceCapacity);
8 final int availableReduceSlots =
9 Math.min((trackerCurrentReduceCapacity - trackerRunningReduces), 1);
10 boolean exceededReducePadding = false;
11 if (availableReduceSlots > 0) {
12 exceededReducePadding = exceededPadding(false, clusterStatus,
13 trackerReduceCapacity);
同理，这部分用来计算是否给Reduce task留有足够的slot去执行失败的和推测执行的Reduce task。
1 synchronized (jobQueue) {
2 for (JobInProgress job : jobQueue) {
3 if (job.getStatus().getRunState() != JobStatus.RUNNING ||
4 job.numReduceTasks == 0) {
5 continue;
6 }
7
8 Task t =
9 job.obtainNewReduceTask(taskTrackerStatus, numTaskTrackers,
10 taskTrackerManager.getNumberOfUniqueHosts()
11 );
12 if (t != null) {
13 assignedTasks.add(t);
14 break;
15 }
16
17 // Don't assign reduce tasks to the hilt!
18 // Leave some free slots in the cluster for future task-failures,
19 // speculative tasks etc. beyond the highest priority job
20 if (exceededReducePadding) {
21 break;
22 }
23 }
24 }
25 }
这部分用来分配Reduce task。可以看到，与分配Map task时用的双层for循环不同，分配Reduce task的时候是单层for循环，因为每次只分配一个Reduce task。Reduce task分配优先级为：
1）、从nonRunningReduces中选择
2）、从runningReduces选择一个task为其启动推测任务
最后，如果findNewReduceTask ()方法返回值为-1，则表示没有找到合适的Reduce task。否则返回值表示该Reduce task在JobInProgress的reduces[]数组中的下标。
1 if (LOG.isDebugEnabled()) {
2 LOG.debug("Task assignments for " + taskTrackerStatus.getTrackerName() + " --> " +
3 "[" + mapLoadFactor + ", " + trackerMapCapacity + ", " +
4 trackerCurrentMapCapacity + ", " + trackerRunningMaps + "] -> [" +
5 (trackerCurrentMapCapacity - trackerRunningMaps) + ", " +
6 assignedMaps + " (" + numLocalMaps + ", " + numNonLocalMaps +
7 ")] [" + reduceLoadFactor + ", " + trackerReduceCapacity + ", " +
8 trackerCurrentReduceCapacity + "," + trackerRunningReduces +
9 "] -> [" + (trackerCurrentReduceCapacity - trackerRunningReduces) +
10 ", " + (assignedTasks.size()-assignedMaps) + "]");
11 }
12
13 return assignedTasks;
最后返回分配给该TaskTracker的task集合。
说一下JobInProgress中与分配任务相关的重要数据结构：
1 Map<Node, List<TaskInProgress>> nonRunningMapCache：Node与未运行的TIP集合映射关系，通过作业的InputFormat可直接获取
2 Map<Node, Set<TaskInProgress>> runningMapCache：Node与运行的TIP集合映射关系，一个任务获得调度机会，其TIP便会添加进来
3 final List<TaskInProgress> nonLocalMaps：non-local(没有输入数据，InputSplit为空)且未运行的TIP集合
4 final SortedSet<TaskInProgress> failedMaps：按照Task Attempt失败次数排序的TIP集合
5 Set<TaskInProgress> nonLocalRunningMaps：non-local且正在运行的TIP集合
6 Set<TaskInProgress> nonRunningReduces：等待运行的Reduce集合
7 Set<TaskInProgress> runningReduces：正在运行的Reduce集合
关于Map task本地性的实现：
JobInProgress中的数据结构nonRunningMapCache体现了本地性，其中记录的是node与该node上待运行的Map task(TaskInProgress)集合。这个数据结构在JobInProgress中的createCache()中创建：
1 private Map<Node, List<TaskInProgress>> createCache(
2 TaskSplitMetaInfo[] splits, int maxLevel)
3 throws UnknownHostException {
4 Map<Node, List<TaskInProgress>> cache =
5 new IdentityHashMap<Node, List<TaskInProgress>>(maxLevel);
6
7 Set<String> uniqueHosts = new TreeSet<String>();
8 for (int i = 0; i < splits.length; i++) {
9 String[] splitLocations = splits[i].getLocations();
10 if (splitLocations == null || splitLocations.length == 0) {
11 nonLocalMaps.add(maps[i]);
12 continue;
13 }
14
15 for(String host: splitLocations) {
16 Node node = jobtracker.resolveAndAddToTopology(host);
17 uniqueHosts.add(host);
18 LOG.info("tip:" + maps[i].getTIPId() + " has split on node:" + node);
19 for (int j = 0; j < maxLevel; j++) {
20 List<TaskInProgress> hostMaps = cache.get(node);
21 if (hostMaps == null) {
22 hostMaps = new ArrayList<TaskInProgress>();
23 cache.put(node, hostMaps);
24 hostMaps.add(maps[i]);
25 }
26 //check whether the hostMaps already contains an entry for a TIP
27 //This will be true for nodes that are racks and multiple nodes in
28 //the rack contain the input for a tip. Note that if it already
29 //exists in the hostMaps, it must be the last element there since
30 //we process one TIP at a time sequentially in the split-size order
31 if (hostMaps.get(hostMaps.size() - 1) != maps[i]) {
32 hostMaps.add(maps[i]);
33 }
34 node = node.getParent();
35 }
36 }
37 }
38
39 // Calibrate the localityWaitFactor - Do not override user intent!
40 if (localityWaitFactor == DEFAULT_LOCALITY_WAIT_FACTOR) {
41 int jobNodes = uniqueHosts.size();
42 int clusterNodes = jobtracker.getNumberOfUniqueHosts();
43
44 if (clusterNodes > 0) {
45 localityWaitFactor =
46 Math.min((float)jobNodes/clusterNodes, localityWaitFactor);
47 }
48 LOG.info(jobId + " LOCALITY_WAIT_FACTOR=" + localityWaitFactor);
49 }
50
51 return cache;
52 }
在这个方法中，根据split所在的node，将与该分片对应的Map Task(TaskInProgress)和Node添加到该数据结构中。当选择未运行的Map Task时，只要从该数据结构中查找与该结点对应的任务即可实现本地性。
本文基于hadoop1.2.1
如有错误，还请指正
参考文章：《Hadoop技术内幕 深入理解MapReduce架构设计与实现原理》 董西成