Add documentation for dynamic allocation (without configs)

docs/job-scheduling.md

@@ -56,6 +56,112 @@ the same RDDs. For example, the [Shark](http://shark.cs.berkeley.edu) JDBC serve
 queries. In future releases, in-memory storage systems such as [Tachyon](http://tachyon-project.org) will
 provide another approach to share RDDs.
 
+## Dynamic Resource Allocation
+
+Spark 1.2 introduces the ability to dynamically scale the set of cluster resources allocated to
+your application up and down based on the workload. This means that your application may give
+resources back to the cluster if they are no longer used and request them again later when there
+is demand. This feature is particularly useful if multiple applications share resources in your
+Spark cluster. If a subset of the resources allocated to an application becomes idle, it can be
+returned to the cluster's pool of resources and acquired by other applications. In Spark, dynamic
+resource allocation is performed on the granularity of the executor and can be enabled through
+`spark.dynamicAllocation.enabled`.
+
+This feature is currently disabled by default and available only on [YARN](running-on-yarn.html).
+A future release will extend this to [standalone mode](spark-standalone.html) and
+[Mesos coarse-grained mode](running-on-mesos.html#mesos-run-modes). Note that although Spark on
+Mesos already has a similar notion of dynamic resource sharing in fine-grained mode, enabling
+dynamic allocation allows your Mesos application to take advantage of coarse-grained low-latency
+scheduling while sharing cluster resources efficiently.
+
+Lastly, it is worth noting that Spark's dynamic resource allocation mechanism is cooperative.
+This means if a Spark application enables this feature, other applications on the same cluster
+are also expected to do so. Otherwise, the cluster's resources will end up being unfairly
+distributed to the applications that do not voluntarily give up unused resources they have
+acquired.
+
+### Configuration and Setup
+
+All configurations used by this feature live under the `spark.dynamicAllocation.*` namespace.
+To enable this feature, your application must set `spark.dynamicAllocation.enabled` to `true` and
+provide lower and upper bounds for the number of executors through
+`spark.dynamicAllocation.minExecutors` and `spark.dynamicAllocation.maxExecutors`. Other relevant
+configurations are described on the [configurations page](configuration.html) and in the subsequent
+sections in detail.
+
+Additionally, your application must use an external shuffle service (described below). To enable
+this, set `spark.shuffle.service.enabled` to `true`. In YARN, this external shuffle service is
+implemented in `org.apache.spark.yarn.network.YarnShuffleService` that runs in each `NodeManager`
+in your cluster. To start this service, follow these steps:
+
+1. Build Spark with the [YARN profile](building-spark.html). Skip this step if you are using a
+pre-packaged distribution.
+2. Locate the `spark-<version>-yarn-shuffle.jar`. This should be under
+`$SPARK_HOME/network/yarn/target/scala-<version>` if you are building Spark yourself, and under
+`lib` if you are using a distribution.
+2. Add this jar to the classpath of all `NodeManager`s in your cluster.
+3. In the `yarn-site.xml` on each node, add `spark_shuffle` to `yarn.nodemanager.aux-services`,
+then set `yarn.nodemanager.aux-services.spark_shuffle.class` to
+`org.apache.spark.yarn.network.YarnShuffleService`. Additionally, set all relevant
+`spark.shuffle.service.*` [configurations](configuration.html).
+4. Restart all `NodeManager`s in your cluster.
+
+### Resource Allocation Policy
+
+On a high level, Spark should relinquish executors when they are no longer used and acquire
+executors when they are needed. Since there is no definitive way to predict whether an executor
+that is about to be removed will run a task in the near future, or whether a new executor that is
+about to be added will actually be idle, we need a set of heuristics to determine when to remove
+and request executors.
+
+#### Request Policy
+
+A Spark application with dynamic allocation enabled requests additional executors when it has
+pending tasks waiting to be scheduled. This condition necessarily implies that the existing set
+of executors is insufficient to simultaneously saturate all tasks that have been submitted but
+not yet finished.
+
+Spark requests executors in rounds. The actual request is triggered when there have been pending
+tasks for `spark.dynamicAllocation.schedulerBacklogTimeout` seconds, and then triggered again
+every `spark.dynamicAllocation.sustainedSchedulerBacklogTimeout` seconds thereafter if the queue
+of pending tasks persists. Additionally, the number of executors requested in each round increases
+exponentially from the previous round. For instance, an application will add 1 executor in the
+first round, and then 2, 4, 8 and so on executors in the subsequent rounds.
+
+The motivation for an exponential increase policy is twofold. First, an application should request
+executors cautiously in the beginning in case it turns out that only a few additional executors is
+sufficient. This echoes the justification for TCP slow start. Second, the application should be
+able to ramp up its resource usage in a timely manner in case it turns out that many executors are
+actually needed.
+
+#### Remove Policy
+
+The policy for removing executors is much simpler. A Spark application removes an executor when
+it has been idle for more than `spark.dynamicAllocation.executorIdleTimeout` seconds. Note that,
+under most circumstances, this condition is mutually exclusive with the request condition, in that
+an executor should not be idle if there are still pending tasks to be scheduled.
+
+### Graceful Decommission of Executors
+
+Before dynamic allocation, a Spark executor exits either on failure or when the associated
+application has also exited. In both scenarios, all state associated with the executor is no
+longer needed and can be safely discarded. With dynamic allocation, however, the application
+is still running when an executor is explicitly removed. If the application attempts to access
+state stored in or written by the executor, it will have to perform a recompute the state. Thus,
+Spark needs a mechanism to decommission an executor gracefully by preserving its state before
+removing it.
+
+This requirement is especially important for shuffles. During a shuffle, the Spark executor first
+writes its own map outputs locally to disk, and then acts as the server for those files when other
+executors attempt to fetch them. In the event of stragglers, which are tasks that run for much
+longer than their peers, dynamic allocation may remove an executor before the shuffle completes,
+in which case the shuffle files written by that executor must be recomputed unnecessarily.
+
+The solution for preserving shuffle files is to use an external shuffle service, also introduced
+in Spark 1.2. This service refers to a long-running process that runs on each node of your cluster
+independently of your Spark applications and their executors. If the service is enabled, Spark
+executors will fetch shuffle files from the service instead of from each other. This means any
+shuffle state written by an executor may continue to be served beyond the executor's lifetime.
 
 # Scheduling Within an Application