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Section: Scientific Foundations

Migration and resilience

Participants : Yves Denneulin, Jean-François Méhaut.

Making a distributed system reliable has been and remains an active research domain. Nonetheless this has not so far lead to results usable in an intranet or federal architecture for computing. Most propositions address only a given application or service. This may be due to the fact that until clusters and intranet architectures arose, it was obvious that client and server nodes were independent. So, a fault or a predictable disconnection on most of the nodes didn't lead to a complete failure of the system. This is not the case in parallel scientific computing where a fault on a node can lead to a data loss on thousands of other nodes. The reliability of the system is hence a crucial point. MESCAL's work on this topic is based on the idea that each process in a parallel application will be executed by a group of nodes instead of a single node: when the node in charge of a process fails, another in the same group can replace it in a transparent way for the application.

There are two main problems to be solved in order to achieve this objective. The first one is the ability to migrate processes of a parallel, and thus communicating, application without enforcing modifications. The second one is the ability to maintain a group structure in a completely distributed way. The first one relies on a close interaction with the underlying operating systems and networks, since processes can be migrated in the middle of a communication. This can only be done by knowing how to save and replay later all ongoing communications, independently of the communications. Freezing a process to restore it on another node is also an operation that requires collaboration of the operating system and a good knowledge of its internals. The other main problem (keeping a group structure) belongs to the distributed algorithms domain and is of a much higher level nature.

Future work will concern the behavior analysis of checkpoint systems in order to predict precisely critical operations to optimize resource usage (network and disk bandwidth).


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