6. Final Thoughts

6.1 HECToR as a Development Machine

HECToR is clearly an excellent machine for running Castep, even without the efficiency gains made in this project. However there are some features that have made using it as a development machine rather difficult. Chief amongst these are:

• Buffered I/O It is obviously important for performance to buffer I/O, but HECToR does not flush these buffers when the system call flush is invoked, or even on job termination. This made tracking bugs down extremely difficult using Castep's built-in Trace logging, or even adding write statements.

• Out-of-Memory not logged for user When one of HECToR's compute nodes runs out of memory, the Linux OOM module kills a randomly selected process. This may be any process including the Castep job, in which case the job terminates. However the PBS output shows only `exit code 137', indicating that the job was killed, but not why.

• No dedicated benchmarking time In the process of developing and testing the modified Castep, calculations had to be performed on a large number of nodes. Many of these calculations were short-the al3x3 benchmark, for example, takes less than 15mins on 2000 PEs or more-and it would have been very useful to have some time set aside for benchmarking. Perhaps this time could be made available after scheduled downtime.