Summary form only given. The major issue today on cluster and grid computing is the efficient resource management. The evaluation of scheduling strategies is hard because of the generation of jobs under realistic scenario. This is true for rigid jobs (where the number of processors is fixed) and even more for moldable ones. We present an approach to generate realistic workloads for this kind of jobs. The model we propose is based on the analysis of one year of utilization of the I-cluster, a 225 processors cluster. From this log we extract a typical load for this kind of parallel machines and introduce a way to generate synthetic realistic workloads in an automatic way. This work was done as a way to test scheduling strategies taking into account both rigid and moldable jobs so as the workload generator may handle moldable jobs.