Team AlGorille

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Section: Scientific Foundations

Keywords : reproductibility, extendability, applicability, simulations, emulations, experiments in situ.

Experimental Validation

Participants : Malek Cherier, Xavier Delaruelle, Emmanuel Jeannot, Martin Quinson.

An important issue for the research on complex systems such as grids is to validate the obtained results. This validation constitutes a scientific challenge by itself since we have to validate models, their adequation to reality and the algorithms that we design inside these models. Whereas mathematical proofs establish soundness within such a context, the overall validation must be done by experiments. A successful experiment shows the validity of both the algorithm and the modeling at the same time. But, if the algorithm does not provide the expected result or performance, this might be due to several factors: a faulty modeling, a weak design, or a bad implementation.

Experimental validation of grid systems is a particularly challenging issue. Such systems will be large, rapidly changing, shared and severely protected. Naive experiments on real platforms will usually not be reproducible, while the extensibility and applicability of simulations and emulations will be very difficult to achieve. These difficulties imply the study phases through modeling, algorithm design, implementation, tests and experiments. The test results will reveal precious for a subsequent modeling phase, complementing the process into a feedback loop.

In addition to this idea of validating the whole (modeling, design and implementation) in our research we are often restricted by a lack of knowledge: the systems that we want to describe might be too complex; some components or aspects might be unknown or the theoretical investigations might not yet be sufficiently advanced to allow for provable satisfactory solutions of problems.

We think that an experimental validation is a valuable completion of theoretical results on protocol and algorithm behavior. The focus of algorithmic research being upon performance, the main experiments we are concerned with are performance evaluation. To our opinion, such experiments should fulfill the following properties:

reproducibility:

Experimental settings must be designed and described such that they are reproducible by others and must give the same result with the same input.

extensibility:

A report on a scientific experiment concerning performance of an implementation on a particular system is only of marginal interest if it is simply descriptive and does not point beyond the particular setting in which it is performed. Therefore, the design of an experiment must target possible comparisons with passed and (in particular) future work, extensions to more and other processors, larger data sets, different architectures and alike. Several dimensions have to be taken into account: scalability, portability, prediction and realism.

applicability:

Performance evaluation should not be a goal in fine but should result in concrete predictions of the behavior of programs in the real world. However, as the set of parameters and conditions is potentially infinite, a good experiment campaign must define realistic parameters for platforms, data sets, programs, applications, etc. and must allow for an easy calibration.

revisability:

When an implementation does not perform as expected, it should be possible to identify the reasons, be they caused by the modeling, the algorithmic design, the particular implementation and/or the experimental environment. Methodologies that help to explain mispredictions and to indicate improvements have to be developed.


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