Process Migration

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Process Migration
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Process Migration

• Transfer of sufficient amount of the state of a
  process from one machine to another
• The process executes on the target machine

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Motivation

• Load sharing
• Move processes from heavily loaded to lightly
  load systems
• Load can be balanced to improve overall
  performance
• Communications performance
• Processes that interact intensively can be moved
  to the same node to reduce communications cost
• May be better to move process to where the data
  reside when the data is large

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Motivation

• Availability
• Long-running process may need to move because the
  machine it is running on will be down
• Utilizing special capabilities
• Process can take advantage of unique hardware or
  software capabilities

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Mechanism

• A number of issues need to be addressed in
  designing a process migration facility.
• Among them are
• Who initiates the migration?
• What portion of the process is migrated?
• What happens to outstanding messages and signals?

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Initiation of Migration

• Operating system
• When goal is load balancing
• Process
• When goal is to reach a particular resource

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What is Migrated?

• Must destroy the process on the source system and
  create it on the target system
• Process control block and any links must be moved

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What is Migrated?

• Eager (all)Transfer entire address space
• No trace of process is left behind
• If address space is large and if the process does
  not need most of it, then this approach my be
  unnecessarily expensive

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What is Migrated?

• Precopy Process continues to execute on the
  source node while the address space is copied
• Pages modified on the source during precopy
  operation have to be copied a second time
• Reduces the time that a process is frozen and
  cannot execute during migration

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What is Migrated?

• Eager (dirty) Transfer only that portion of the
  address space that is in main memory and have
  been modified
• Any additional blocks of the virtual address
  space are transferred on demand
• The source machine is involved throughout the
  life of the process

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What is Migrated?

• Copy-on-reference Pages are only brought over on
  reference
• Variation of eager (dirty)
• Has lowest initial cost of process migration

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What is Migrated?

• Flushing Pages are cleared from main memory by
  flushing dirty pages to disk
• Relieves the source of holding any pages of the
  migrated process in main memory

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Code-migration

• Weak-mobility when a process is transferred to
  another node, the process is restarted from the
  beginning.
• Strong-mobility when a process that is
  currently executing is transferred and is allowed
  to continue from where it stopped.
• Clearly Strong-mobility is much harder.

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IBMs AIX DOS

• The following sequence of events happen
• When a process decides to migrate itself, it
  selects a target machine and send a remote
  tasking message. The message carries part of the
  process image and open file information.
• At the receiving site, a kernel server process
  forks a child, giving it this information.

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IBMs AIX DOS

• The new process pulls the date, environment,
  arguments, or stack information as needed to
  complete its operation. Program text is copied
  over if it is dirty or demand paged from the
  global file system if it is clean.
• The origination process is signaled on the
  completion of the migration. This process sends a
  final done message to the new process and
  destroys itself.

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Negotiation of Migrationin Charlotte

• Migration policy (which process, when and to
  where) is responsibility of Starter utility.
• Starter utility is also responsible for long-term
  scheduling and memory allocation.
• Decision to migrate must be reached jointly by
  two Starter processes (one on the source and one
  on the destination).

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Eviction

• System evict a process that has been migrated to
  it
• If a workstation is idle, process may have been
  migrated to it
• Once the workstation is active, it may be
  necessary to evict the migrated processes to
  provide adequate response time

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Eviction in Sprite

• A monitor process at each node keeps track of the
  current load. If activity is detected on the
  local console. Foreign processes are evicted.
• If a process is evicted, it is migrated back to
  its home node (and could be subsequently sent
  elsewhere).

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• Although it may take some time to evict all
  selected foreign processes, all such processes
  are suspended immediately.
• The entire address space of an evicted process is
  transferred to the home node. The time to evict a
  process and migrate it back to its home node may
  be reduced substantially by retrieving the memory
  image of the process from the foreign host.

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• Distributed Process Management