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From "Harsh J (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-278) Proposal for redesign/refactoring of the JobTracker and TaskTracker
Date Mon, 23 Apr 2012 09:35:34 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-278?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13259481#comment-13259481
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Harsh J commented on MAPREDUCE-278:
-----------------------------------

[~yixiaohua] - Did you accidentally upload that file here? This JIRA is closed and the patch
you've attached appears irrelevant. Please reupload to the right JIRA instead.
                
> Proposal for redesign/refactoring of the JobTracker and TaskTracker
> -------------------------------------------------------------------
>
>                 Key: MAPREDUCE-278
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-278
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>            Reporter: Arun C Murthy
>            Assignee: Sharad Agarwal
>         Attachments: Job_Tracker_FSM.pdf, TestUTF8AndStringGetBytes.java, mapred_as_dfa.patch
>
>
> During discussions on HADOOP-815 wrt some hard-to-maintain code on the JobTracker we
all agreed that the current state-of-affairs there is brittle and merits some rework.
> Case in point: there are back-calls from TaskInProgress to JobTracker and from JobInProgress
to JobTracker which mean that synchronization is quite involved and brittle, leading to issues
like HADOOP-600. Also one is forced to lock several data-structures individually before certain
operations (taskTrackers, trackerExpiryQueue, jobs etc.)
> Hence I'd like to present some early thoughts (which have undergone a quick iteration)
on how we could do slightly better by a bit of redesign/refactoring, also during discussions
with Owen on the same we agreed that HADOOP-554 is an integral part along the same direction...
and I also feel that a good candidate to be done along with this is HADOOP-398 (mapred package
refactoring).
> Context:
> ---------
> a) The unit of communication between the JobTracker & TaskTracker is a 'task'.
> b) Due to (a) the JobTracker maintains a bunch of information related on the 'taskid'
i.e. taskidToTipMap, taskidToTrackerMap etc. and hence we need to update the JobTracker's
data-structures via back-calls from TaskInProgress & JobInProgress where the context is
available (complete/failed task, already-completed task etc.)
> c) This implies that we have a fairly elaborate and hard to maintain locking structures
and also some redundant information in the JobTracker; making it harder to maintain.
> Overall at both the JobTracker & TaskTracker the concept of a 'job' is overshadowed
by the 'task'; which I propose we fix.
> Proposal:
> ----------
> Here is the main flow of control:
> JobTracker -> JobInProgress -> TaskInProgress -> task_attempt
> The main idea is to break the existing nexus between the JobTracker & TaskInProgress/taskid
by (I've put code for illustrative purposes only, and ignored pieces irrelevant to this discussion):
> a) Making the 'job' the primary unit of communication between JobTracker & TaskTracker.
> b) TaskTrackerStatus now looks like this: 
>   class TaskTrackerStatus {
>     List<JobStatus> jobStatuses; // the status of the 'jobs' running on a TaskTracker
>     String getTrackerName();
>   }
>   class JobStatus {
>     List<TaskStatus> taskStatuses; // the status of the 'tasks' belonging to a
job
>     JobId getJobId();
>   }
> c) The JobTracker maintains only a single map of jobid -> JobInProgress, and mapping
from taskTracker -> List<JobInProgress>
>   Map<JobId, JobInProgress> allJobs;
>   Map<String, List<JobInProgress>> trackerToJobsMap;
> d) The JobTracker delegates a bunch of responsibilities to the JobInProgress to reflect
the fact the primary 'concept' in map/reduce is the 'job', thus empowering the JobInProgress
class:
>   class JobInProgress {
>     TaskInProgress[] mapTasks;
>     TaskInProgress[] reduceTasks;
>     
>     Map<String, List<TaskInProgress>> trackerToTasksMap; // tracker ->
tasks running
>     Map<String, List<TaskAttempt>> trackerToMarkedTasksMap; // tracker ->
completed (success/failed/killed) task-attempt, 
>                                                                                     
    //                but the tracker doesn't know it yet
>     void updateStatus(JobStatus jobStatus);
>     MapOutputLocation[] getMapOutputLocations(int[] mapTasksNeeded, int reduce);
>     TaskAttempt getTaskToRun(String taskTracker);
>     List<TaskTrackerAction> getTaskToKill(String taskTracker);
>   }
>   
> d) On receipt of TaskTrackerStatus from a tracker, the processeing of heartbeat looks
like this:
>   for (JobStatus jobStatus : taskTrackerStatus.getJobStatuses()) {
>    JobInProgress job = allJobs.get(jobId);
>    synchronized (job) {
>      job.updateStatus(jobStatus);
>      return (HeartbeatResponse(repsonseId,
>                                job.getTaskAttemptToRun(trackerName), 
>                                job.getTaskToKill(trackerName)
>                               ));
>     }
>   }
>    
> The big change is that the JobTracker delegates a lot of responsibility to the JobInProgress,
we get away from all the complicated synchronization constructs: simply lock the JobInProgress
object at all places via allJobs/trackerToJobsMap and we are done. This also enhances throughput
since mostly we will not need to lock up the JobTracker (even in the heartbeat loop); locking
the JobInProgress or the 2 maps is sufficient in most cases... thus enhance the inherent parallelism
of the JobTracker's inner loop (processing heartbeat) and provide better response when multiple
jobs are running on the cluster. 
> Hence the JobInProgress is responsible for maintaining it's TaskInProgress'es which in
turn are completely responsible for the TaskAttempt`s, the JobInProgress also provides sufficient
information as and when needed to the JobTracker to schedule jobs/tasks and the JobTracker
is blissfully unaware of the innards of jobs/tasks.
> -*-*-
> I hope to articulate more a general direction towards an improved and maintainable 'mapred'
and would love to hear out how we can improve and pitfalls to avoid... lets discuss. We could
take this piecemeal an implement or at one go...
> Last, not least; I propose that while we are at this we redo the nomenclature a bit:
> JobInProgress -> Job
> TaskInProgress -> Task
> taskid -> replace with a new TaskAttempt
> this should help clarify each class and it's roles.
> Of course we will probably need a separate org.apache.hadoop.mapred.job.Task v/s org.apache.hadoop.mapred.task.Task
which is why I feel HADOOP-554 (refactoring of mapred packages) would be very important to
get a complete, coherent solution.
> Thoughts?

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