Hi, Firstly, I am talking about Hadoop 1.0. Please note that in Hadoop 2.x and trunk, the Mapreduce framework is completely revamped to Yarn ( http://hadoop.apache.org/docs/current/hadoop-yarn/hadoop-yarn-site/YARN.html) and you may need to look at different interfaces for building your own scheduler.
In 1.0, the primary function of the TaskScheduler is the assignTasks method. Given a TaskTracker object as input, this method figures out how many free map and reduce slots exist in that particular tasktracker and selects one or more task that can be scheduled on it. Since task selection is the primary responsibility and the granularity is at a task level, the class is called TaskScheduler. The method of choosing a job and then a task within the job is customised by the different schedulers already present in Hadoop. Also, the core logic of selecting a map task with data locality optimizations is not implemented in the schedulers per se, but they rely on the JobInProgress object in MapReduce framework for achieving the same. To implement your own Scheduler, it may be best to look at the sources of existing schedulers: JobQueueTaskScheduler, CapacityTaskScheduler or FairScheduler. In particular, the last two are in the contrib modules of mapreduce, and hence will be fairly independent to follow. Their build files will also tell you how to resolve any compile problems like the one you are facing. Thanks Hemanth On Thu, Dec 27, 2012 at 4:10 PM, Yaron Gonen <yaron.go...@gmail.com> wrote: > Hi, > If I understand correctly, the job scheduler (why is the class called > TaskScheduler?) is responsible for assigning the task whose split is as > close as possible to the tasktacker. > Meaning that the job scheduler is responsible to two things: > > 1. Selecting a job. > 2. Once a job is selected, assign the closest task to the tasktracker > that send the heartbeat. > > Is this correct? > > I want to write my own job scheduler to change the logic above, but it > says The type TaskScheduler is not visible. > How can I write my own scheduler? > > thanks >
