I/O scalability boost for the next generation of Earth system models: IFS-XIOS integration as a case study

Earth system models (ESMs) have increased the spatial resolution to achieve more accurate solutions, producing an enormous amount of data. However, some ESMs use inefficient sequential I/O schemes that do not scale well when many parallel resources are used. This issue is typically addressed by adopting scalable parallel I/O solutions. This work analyzes and improves the I/O process of the Integrated Forecasting System (IFS), one of the most important atmospheric models used in Europe. IFS can use two different output schemes: the MF I/O server and an inefficient sequential I/O scheme. The latter is the only scheme that can be used by non ECMWF users. This is the case of EC-Earth, a global coupled climate model that uses IFS as its atmospheric component. In recent experiments of EC-Earth using the T511L91-ORCA025L75 configuration, the I/O part of IFS represented about the 30% of the total execution time. In addition, EC-Earth simulations have to run post-processing tasks that perform costly operations. Therefore, it is presented an easy-to-use development that integrates an asynchronous parallel I/O server called the XML I/O Server (XIOS) into IFS. XIOS offers a series of features that are especially targeted to climate models: netCDF format data, online diagnostics and CMORized data. Thus, it will be possible to shorten the critical path of EC-Earth experiments by concurrently running the post-processing task along the EC-Earth execution. Moreover, a profiling analysis is done to evaluate the new computational performance of the IFS-XIOS integration, proving that the integration itself is not optimal. Thus, it is necessary to optimize it to increase both computational performance and efficiency. As a consequence, different HPC optimization techniques, such as computation and communication overlap, are applied in the integration to minimize the I/O overhead in the IFS execution. The results show that the use of XIOS in IFS to output data achieves an adequate performance. This new parallel scheme has reduced significantly the execution time of the original sequential scheme. In one of the tests, XIOS is able to output 3.2 TB of data in only two and a half minutes of overhead. Furthermore, when the cost of converting GRIB to netCDF files is taken into account, the use of XIOS causes that the overall execution is three times faster.