Chapter 6

Mechanisms for Process Migration

Introduction

Load balancing can be either nonpreemptive or preemptive. In nonpreemptive dynamic scheduling called dynamic task placement, the tasks are eligible for redistribution only before they start to execute. In preemptive scheduling, called process migration, a task may be relocated after execution begins. The steps involved include :

Suspending the migrating process on the source host
Transferring the process along with the current state to the target host
Resuming the execution of the process on the target host

Process migration policies are similar to those of dynamic task placement policies. However to avoid thrashing, an eligible task must be :

Larger than a threshold value
Has been executed on the source machine longer than a threshold value
Belongs to the set of processes that have been transferred least among those residing on the source machine

Both the above methods have a basic overhead associated with them, but process migration has the additional overhead of transferring the source machine state to the target machine. A typical process state consists of virtual memory information, open files, message buffers, state and environment data. A major task is to transfer virtual memory information which can be done in the following ways:

Entire virtual memory transfer
Pre copy transfer : Set up a pipeline so that process execution and state transformation take place simultaneously. Then afetr a migration decision is made, the parallel processing and transfer go on. The transfer can go on till some threshold of the number of dirty pages is reached. Finally the process is suspended and the cujrrent dirty pages are transferred.
Copy on reference transfer : At migration time, the process and its state, with the exception of virtual memory are transferred to target where execution resaumes while required pages are transferred from soure on demand.
Enhanced copy on reference transfer : Here both source and target are assumed to share a file server. This resembles the previous method

Is it worthwhile?

The process state can become very large making the migration, a very expensive operation. If a parent process spawns child process, then there is an additional problem if the parent migrates without the children knowing the new location. However if augmented with dynamic task placement, there could be improvements.

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