How YARN Changed Hadoop Job Scheduling

Scheduling means different things depending on the audience. To many in the business world, scheduling is synonymous with workflow management. Workflow management is the coordinated execution of a collection of scripts or programs for a business workflow with monitoring, logging and execution guarantees built in to a WYSIWYG editor. Tools like Platform Process Manager come to mind as an example. To others, scheduling is about process or network scheduling. In the distributed computing world, scheduling means job scheduling, or more correctly, workload management.

Workload management is not only about how a specific unit of work is submitted, packaged and scheduled, but it's also about how it runs, handles failures and returns results. The HPC definition is fairly close to the Hadoop definition of scheduling. One interesting way that HPC scheduling and resource management cross paths is within the Hadoop on Demand project. The Torque resource manager and Maui Meta Scheduler both were used for scheduling in the Hadoop on Demand project during Hadoop's early days at Yahoo.

This article compares and contrasts the historically robust field of HPC workload management with the rapidly evolving field of job scheduling happening in Hadoop today.

Both HPC and Hadoop can be called distributed computing, but they diverge rapidly architecturally. HPC is a typical share-everything architecture with compute nodes sharing common storage. In this case, the data for each job has to be moved to the node via the shared storage system. A shared storage layer makes writing job scripts a little easier, but it also injects the need for more expensive storage technologies. The share-everything paradigm also creates an ever-increasing demand on the network with scale. HPC centers quickly realize they must move to higher speed networking technology to support parallel workloads at scale.

Hadoop, on the other hand, functions in a share-nothing architecture, meaning that data is stored on individual nodes using local disk. Hadoop moves work to the data and leverages inexpensive and rapid local storage (JBOD) as much as possible. A local storage architecture scales nearly linearly due to the proportional increase in CPU, disk and I/O capacity as node count increases. A fiber network is a nice option with Hadoop, but two bonded 1GbE interfaces or a single 10GbE in many cases is fast enough. Using the slowest practical networking technology provides a net savings to a project budget.

From a Hadoop philosophy, funds really should be allocated for additional data nodes. The same can be said about CPU, memory and the drives themselves. Adding nodes is what makes the entire cluster both more parallel in operation as well as more resistant to failure. The use of mid-range componentry, also called commodity hardware is what makes it affordable.

Until recently, Hadoop itself was a paradigm restricted mainly to MapReduce. Users have attempted to stretch the model of MapReduce to fit an ever-expanding list of use cases well beyond its intended roots. The authors of Hadoop addressed the need to grow Hadoop beyond MapReduce architecturally by decoupling the resource management features built in to MapReduce from the programming model of MapReduce.

The new resource manager is referred to as YARN. YARN stands for Yet Another Resource Negotiator and was introduced in the ASF JIRA MAPREDUCE-279. The YARN-based architecture of Hadoop 2 allows for alternate programming paradigms within Hadoop. The architecture uses a master node dæmon called a Resource Manager consisting of two parts, a scheduler and Application Manager.

The scheduler is commonly called a pure scheduler in that it is only managing resource availability from the node manager on the data nodes. It also enforces scheduling policy as it is defined in the configuration files. The scheduler functions to schedule containers that are customizable collections of resources.

The Application Master is itself a container, albeit a special one, sometimes called container 0. The Application Master is responsible for launching subsequent containers as required by the job. The second part of the Resource Manager, called the Application Manager, receives job submissions and manages launching the Application Master. The Application Manager handles failures of the Application Master, while the Application Master handles failures of job containers. The Application Master then is really an application-specific container charged with management of containers running the actual tasks of the job.

Figure 1. YARN-Based Architecture of Hadoop 2

Refactoring of resource management from the programming model of MapReduce makes Hadoop clusters more generic. Under YARN, MapReduce is one type of available application running in a YARN container. Other types of applications now can be written generically to run on YARN including well-known applications like HBase, Storm and even MPI applications. The progress of MPI support can be seen in the Hamster project and a project called mpich2-yarn available on GitHub. YARN then moves from being a scheduler to an operating system for the Hadoop supporting multiple applications on a distributed architecture.

Architecturally, HPC workload management has many similarities to Hadoop workload management. Depending on the HPC workload management technology used, there is a set of master nodes containing cluster-controlling dæmons for accepting and scheduling jobs. The master node(s) in many cases contains special configurations including sharing of important cluster data via networked storage to eliminate SPOF of master services. On the worker node side, there exists one or more dæmons running to accept jobs and report resource availability to the master node dæmons. Technologies from HPC, like Platform LSF and PBS Professional as well as other open-source variants like SLURM and Torque, are commonly seen in HPC.

These technologies are much older than Hadoop, and in terms of scheduling policy, they are more mature. They tend to share some basic tenets of scheduling policy that the Hadoop community is either in the process of addressing or has already.

Table 1. Comparison of Scheduling Options

Policy or Feature HPC Hadoop
FIFO Available Available
Fair Share Available Available
Time-Based Policies Available Technology Gap
Preemption Available Available
Exclusive Placement Available Technology Gap
Custom Algorithms Available Available
SLA- or QoS-Based Available Technology Gap
Round-Robin Available Technology Gap
Static and Dynamic Resources Available Available
Node Labeling Available Coming Soon
Custom Resources Available Technology Gap

First-In First-Out Scheduling

Many times this is the default policy used when a workload manager is first installed. As the name suggests, FIFO operates like a line or queue at a movie theatre.

Fair Share

Fair Share is a scheduling policy that attempts to allocate cluster resources fairly to jobs based upon a fixed number of shares per user or group. Fair share is implemented differently based upon the exact cluster resource management software used, but most systems have the concept of ordering jobs to be run in an attempt to even out the use of resources for all users. The specific ordering can be based upon a fixed number of shares or a percentage capacity of resources along with policies for an individual queue or a hierarchy of queues.

Time-Based Policies

Time-based policies come in a few different varieties. Queue-level time-based policies might be used to alter the configuration of a queue based upon time of day including allowing jobs to be submitted (enqueued) but not dispatched to nodes. Time-based policies enable concepts like using a cluster for a specific workload during business hours and an alternate workload overnight. Other time-based policies include dedicating the entire cluster or portion of a cluster for a specific use for a length of time. Additionally, draining a cluster of submitted jobs for maintenance windows is common.

______________________

Adam Diaz is a longtime Linux geek and fan of distributed/parallel systems. Adam cut his teeth working for companies like Platform Computing, Altair Engineering and a handful of startups.

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