Task migration and its issues

Task Migration

  • It is the movement of an executing task from one host processor in distributed computing system to another.
  • Task placement is the selection of a host for a new task and the creation of the task on that host.

Benefits of task Migration

  • Load Balancing: Improve performance for a distributed computing system overall or a distributed application by spreading thee load more evenly over a set of host.
  • Reduction in Communication overhead: By locating on one host a group of tasks with intensive communication amongst them.
  • Resource access: Not all resources are available across the network, a task may need to migrate in order to access a special device, or to satisfy a need for a large amount of physical memory
  • Fault tolerance: Allowing long running processes to survive the planned shutdown or failure of a host.

Steps involved in task migration

  • Suspending the task on the source.
  • Extracting and transmitting the state of the task to destination.
  • Reconstructing the state on the destination
  • Deleting the task on the source and resuming the task’s execution on the destination.

Task Migration Issues

  1. State Transfer
  2. Location transparency
  3. Structure of a migration mechanism

1. State Transfer

  • The cost to support remote execution.
  • Freezing the task.
  • Obtaining and transferring the state.
  • Unfreezing the task.

State Transfer mechanisms

  • Recopying the state: Bulk of the task is copied to the new host before freezing the task.
  • Location transparent file access mechanism
  • Copy on reference: Just copy what is migrated task need for its execution.

3. Location Transparency

  • Task Migration should hide the locations of tasks. Location transparency in principle requires that name (process name, file name) be independent of their locations (host names).
  • Uniform name spacer throughout the system.

3. Structure of a migration mechanism

  • Typically. Here will be interaction between the task migration mechanism, the memory management system, the interprocess communication mechanism and the file system.
  • The mechanism can be designed to be independent for one another so that if one mechanism’s protocol changes; the other’s need not the migration mechanism can be turned off without interfering with other mechanisms.