Process migration
---

- message-based systems (V, Accent) make migration somewhat easier
  than kernel-call-based systems, because some of the state that is
  maintained by the kernel in a kernel-call-based system is maintained
  in a process's address space in a message-based system;



V System [Cheriton]
-------------------

- uses pre-copying, where the process continues executing while its
  memory is transfered from one machine to the other;  then the
  process is frozen and any pages that have been modified are recopied;

- in the initial implementation, the entire memory of the process was
  transferred directly to the target;

- Theimer showed that pre-copying reduces freeze times substantially;

- pre-copying seems most useful in environments like V, that has
  real-time requirements;



Sprite [Ousterhout]
-------------------

- backing files simplify the transfer of the virtual memory image;
  the old machine simply pages out the dirty pages and transfers
  information about the backing files to the target machine;

- since these backing files are stored in the network file system,
  they are accessible throughout the network;

- if the code segment already exists on the new machine, the migrating
  process shares it, as in LOCUS (no need to send from the old machine
  to the new one);

- pages get reloaded in the new machine on demand, using the standard
  virtual memory mechanisms;

- therefore, the process needs to be frozen long enough to write out
  its dirty pages;  this takes longer than in V and Accent, but
  requires less data to be copied than in V and requires no
  participation of the old machine after the migration;

- Sprite achieves transparency in a fashion similar to LOCUS, by
  assigning each process a home node (usually the one where the
  process was created);

- whenever a process invokes a kernel call whose results are
  machine-dependent, the kernel call is forwarded to the home machine;

- system helps to identify idle hosts, but it does not automatically
  migrate processes except for eviction;




LOCUS [Popek]
-------------

- LOCUS checks for a read-only code segment and re-opens it in the new
  machine, rather than copying it from the old machine;

- apart from the optimization above, the process's entire memory image
  is sent to the target machine at migration time;



Accent [Zayas]
--------------

- lazy approach to copying, where the virtual memory image is left on
  the old machine and transfered to the new machine one page at a time
  when page faults occur;

- allows a process to begin execution on the target machine with
  minimal freeze time but introduces many short delays later;

- pages that are not used are not copied at all;

- Zayas found out that for typical programs, usually just 25-50% of a
  process's allocated memory needed to be transferred;

- one disadvantage is that it leaves residual dependencies in the home
  machine;

- a process that migrates more than once may have a list of
  dependencies;