Updates on usage of the Czech national HPC center

The distributed computing of the ATLAS experiment at LHC is using computing resources of the Czech national HPC center IT4Innovations for several years. The submission system is based on ARC-CEs installed at the Czech Tier2 site (praguelcg2). Recent improvements of this system will be discussed here. First, there was a migration of the ARC-CE from version 5 to 6 which improves the reliability and scalability. Shared filesystem built on top of recent sshfs no longer represent performance bottleneck. It was possible to achieve order of magnitude better transfer performance with it. New singularity containers with full software stack can easily fit default resource limits on the IT4I cluster filesystem. System allowing sequential running of payloads in one job was adapted to HPC's environment, improving usage on worker nodes with very high number of cores. Overall, the system provides significant contribution to resources provided by praguelcg2.