Application Checkpointing - Use in Distributed Shared Memory Systems

Use in Distributed Shared Memory Systems

In distributed shared memory, checkpointing is a technique that helps tolerate the errors leading to losing the effect of work of long-running applications. The main property which should be induced by checkpointing techniques in such systems is in preserving system consistency in case of failure. There are two main approaches to checkpointing in such systems: coordinated checkpointing, in which all cooperating processes work together to establish coherent checkpoint; and communication induced (called also dependency induced) independent checkpointing.

It must be stressed that simply forcing processes to checkpoint their state at fixed time intervals is not sufficient to ensure global consistency. Even if we postulate the existence of global clock, the checkpoints made by different processes still may not form a consistent state. The need for establishing a consistent state may force other process to roll back to their checkpoints, which in turn may cause other processes to roll back to even earlier checkpoints, which in the most extreme case may mean that the only consistent state found is the initial state (the so called domino effect).

In the coordinated checkpointing approach, processes must ensure that their checkpoints are consistent. This is usually achieved by some kind of two-phase commit protocol algorithm. In communication induced checkpointing, each process checkpoints its own state independently whenever this state is exposed to other processes (that is, for example whenever a remote process reads the page written to by the local process).

The system state may be saved either locally, in stable storage, or in a distant node's memory.