Task Execution
Applies To: Windows Compute Cluster Server 2003
Job Scheduler dispatches tasks to the compute nodes in the order that they appear in the task list. Dispatching consists of passing the task to a designated node, which can be any of the compute nodes allocated to the job. Unless dependencies have been specified, the tasks are dispatched in the order by which they are received.
For serial tasks, the first two tasks will be dispatched to and run on the designated node (assuming it has two processors), the next two tasks will be dispatched to and run on a second designated node, and the sequence will repeat itself until there are no more tasks or until all the processors in the cluster are being used. Any remaining tasks must wait for the next available processor and run when it becomes available. The following illustration shows this process.
Note
The file server shown on the head node may not actually reside there. It can reside anywhere in the external or internal network. For more information, see Store and Access Data Files. An MSDE server stores the job specifications and user logon credentials. The task ID number, which also contains the job ID number, allows Job Scheduler to keep track of the status of the task as part of the job, displaying both job and task status to the user.
.gif "Serial task flow")
For parallel tasks, execution flow depends on the user application and the software that supports it. For jobs that are run using the Microsoft Message Passing Interface Service (MS MPI), which is included in the Compute Cluster Pack, execution is performed as follows: The MS MPI executable file Mpiexec.exe is started on the designated node. Mpiexec.exe, in turn, starts all the task processes through the node-specific MS MPI Service. If more than one node is required for the task, additional instances of MS MPI, one per node, are spawned before the task processes themselves are started. Parallel task flow is shown in the following figure.
Note
The file server shown on the head node may not actually reside there. It can reside anywhere in the external network. For more information, see Store and Access Data Files. An MSDE server stores the job specifications and user logon credentials.
.gif "Parallel task flow")
In this figure, P0 through P5 represent the processes that are generated, each as part of a single task. This illustration shows the most common case, in which only one process, P0, handles all the standard input and output files.