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Task migration in os

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This document discusses task migration in distributed systems. It defines task migration as the preemptive transfer of a partially executed task to another node. The document outlines the group members working on the task migration project and provides an introduction to terminology used. It then discusses reasons for task migration like load distribution and improving communication. The main issues discussed are state transfer costs, location transparency, migration mechanism structure, and performance impacts.

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TASK MIGRATION Group Members 1 – Imran Khan BSIT-F14-LC-236 2 – Allah Rakha BSIT-F14-LC-241 3 -Sunny Arif BSIT-F14-LC-224 4 –Mirza Khurrum BSIT-F14-LC-249
Introduction  process/task migration as the transfer of partially executed tasks to another node in a distributed system.  in other words, preemptive task transfer. Some references to task migration include the transfer of processes before execution begins, but the most difficult issues are those related to preemption. Terminology:  Task placement: non-preemptive transfer of process that has never run  Task migration: preemptive transfer of process that has been executed  Home node: site where the process originates  Foreign process: process executing on a node other than its home node  Freezing: when a process is migrated, its execution is interrupted (frozen) for the time it takes to move the process to another site.
Introduction Migration is useful for several reasons:  load distribution, to equalize workload due to 1 transient periods of high load at some network nodes (transfer processes from temporarily overloaded node to another node) 2 or in a system where process origination is always unbalanced (a few workstations consistently generate many processes )  to return a workstation to its owner. (migrate a foreign process back to its home node when owner returns)  to improve communication (processes that communicate frequently can be moved to the same node; processes that access resources at a remote site may be moved to that site)
Introduction Two steps in task migration: 1 State transfer: collect all information crucial to process execution and transfer to another machine 2 Unfreeze: after installing the process state at the new site, the process is put in the Ready queue to be scheduled
Issues in Task Migration State Transfer: Issues to consider here include  Cost in processor time, network bandwidth, etc.: When a process is moved to another node, it is frozen until the transfer is complete. There is the cost of a complete context switch, plus transfer, plus possible affect on other processes that might be affected. (Processes that interact with the migrating process may timeout during the interval when it is frozen, and thus could be aborted)  Residual dependencies: These are resources that the original host node must maintain on behalf of the migrated process. For example, a process may transfer pages in virtual memory only as they are referenced; the original host may have to forward messages directed to the process at its original site.
Issues in Task Migration Location Transparency:  Users aren't required to know where a task is executed; there should be no affect on the process. Process names, file names, etc., must be independent of the host machine. A uniform name space is important to allow the process to maintain access to system resources, to continue to be able to communicate with other processes, etc.
Issues in Task Migration Structure of the Migration Mechanism:  Singhal and Shivaratri emphasize the separation of policy from mechanism. Policy decides when/where/why transfer to be done (see discussion of policies earlier) while mechanism is the act of collecting state, sending it elsewhere, etc. As always, there are benefits to separating the two: for example, policies can be changed without affecting mechanisms.
Issues in Task Migration Performance: This is the major drawback to any kind of dynamic task migration facility.
Process Migration Mechanisms Stallings identifies the following issues in designing a process migration facility:  Who initiates the migration?  What part of the process is migrated?  What happens to messages and signals?
Process Migration Mechanisms Migration Initiation  Migration can be initiated either by a module in the operating system, or by the process itself.  OS module is the most common when load sharing is the primary objective of the migration  If the goal is to move closer to resources or other processes, then the migration could be initiated by the process itself.
Process Migration Mechanisms What is Migrated:  The process control block (PCB) is required, but is relatively easy to transfer from one machine to another (especially if the machines are heterogeneous)  Process address space is more difficult. The process will have a number of virtual pages in memory, and the question is whether to some or all, whether to precopy (move pages to new host while execution continues on old host), etc
Process Migration Mechanisms Some possibilities include:  Immediately move all of them (eager transfer)  Move all pages that are dirty, but transfer others on demand only  Move no pages at all until they are referenced  Flush pages to disk, page back in when they are referenced on the remote site
Process Migration Mechanisms Messages and Signals:  Any messages or signals intended for a frozen process are stored at the host until the process transfer is complete  Other messages and signals must be routed from the home site to the new host.

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Task migration in os

  • 1. TASK MIGRATION Group Members 1 – Imran Khan BSIT-F14-LC-236 2 – Allah Rakha BSIT-F14-LC-241 3 -Sunny Arif BSIT-F14-LC-224 4 –Mirza Khurrum BSIT-F14-LC-249
  • 2. Introduction  process/task migration as the transfer of partially executed tasks to another node in a distributed system.  in other words, preemptive task transfer. Some references to task migration include the transfer of processes before execution begins, but the most difficult issues are those related to preemption. Terminology:  Task placement: non-preemptive transfer of process that has never run  Task migration: preemptive transfer of process that has been executed  Home node: site where the process originates  Foreign process: process executing on a node other than its home node  Freezing: when a process is migrated, its execution is interrupted (frozen) for the time it takes to move the process to another site.
  • 3. Introduction Migration is useful for several reasons:  load distribution, to equalize workload due to 1 transient periods of high load at some network nodes (transfer processes from temporarily overloaded node to another node) 2 or in a system where process origination is always unbalanced (a few workstations consistently generate many processes )  to return a workstation to its owner. (migrate a foreign process back to its home node when owner returns)  to improve communication (processes that communicate frequently can be moved to the same node; processes that access resources at a remote site may be moved to that site)
  • 4. Introduction Two steps in task migration: 1 State transfer: collect all information crucial to process execution and transfer to another machine 2 Unfreeze: after installing the process state at the new site, the process is put in the Ready queue to be scheduled
  • 5. Issues in Task Migration State Transfer: Issues to consider here include  Cost in processor time, network bandwidth, etc.: When a process is moved to another node, it is frozen until the transfer is complete. There is the cost of a complete context switch, plus transfer, plus possible affect on other processes that might be affected. (Processes that interact with the migrating process may timeout during the interval when it is frozen, and thus could be aborted)  Residual dependencies: These are resources that the original host node must maintain on behalf of the migrated process. For example, a process may transfer pages in virtual memory only as they are referenced; the original host may have to forward messages directed to the process at its original site.
  • 6. Issues in Task Migration Location Transparency:  Users aren't required to know where a task is executed; there should be no affect on the process. Process names, file names, etc., must be independent of the host machine. A uniform name space is important to allow the process to maintain access to system resources, to continue to be able to communicate with other processes, etc.
  • 7. Issues in Task Migration Structure of the Migration Mechanism:  Singhal and Shivaratri emphasize the separation of policy from mechanism. Policy decides when/where/why transfer to be done (see discussion of policies earlier) while mechanism is the act of collecting state, sending it elsewhere, etc. As always, there are benefits to separating the two: for example, policies can be changed without affecting mechanisms.
  • 8. Issues in Task Migration Performance: This is the major drawback to any kind of dynamic task migration facility.
  • 9. Process Migration Mechanisms Stallings identifies the following issues in designing a process migration facility:  Who initiates the migration?  What part of the process is migrated?  What happens to messages and signals?
  • 10. Process Migration Mechanisms Migration Initiation  Migration can be initiated either by a module in the operating system, or by the process itself.  OS module is the most common when load sharing is the primary objective of the migration  If the goal is to move closer to resources or other processes, then the migration could be initiated by the process itself.
  • 11. Process Migration Mechanisms What is Migrated:  The process control block (PCB) is required, but is relatively easy to transfer from one machine to another (especially if the machines are heterogeneous)  Process address space is more difficult. The process will have a number of virtual pages in memory, and the question is whether to some or all, whether to precopy (move pages to new host while execution continues on old host), etc
  • 12. Process Migration Mechanisms Some possibilities include:  Immediately move all of them (eager transfer)  Move all pages that are dirty, but transfer others on demand only  Move no pages at all until they are referenced  Flush pages to disk, page back in when they are referenced on the remote site
  • 13. Process Migration Mechanisms Messages and Signals:  Any messages or signals intended for a frozen process are stored at the host until the process transfer is complete  Other messages and signals must be routed from the home site to the new host.