The Service Unit (SU) is the charging unit that NCI uses to account for usage of its compute and storage services on an hourly basis.
The concept of a Service Unit (SU) was originally developed for the purpose of supercomputer time accounting, and was based on the core-hours utilised by an application running on a given system. Over time, as systems became more heterogenous and increasingly specialised, methods of charging for resource usage have diverged from the original model based on “one core-hour of execution time”. New HPC offer a more varied spectrum of performance characteristics which are not well represented by the cpu-hour model. The Service Unit has evolved to represent a more generalised description of computational work.
Calculating the SU charge rate between different generations or types of hardware can be difficult as not all applications will see the same benefit, so an averaging approach is taken across several applications. For example, a new piece of hardware that has a charge rate of 2 SU per hour compared to older hardware being charged at 1 SU per hour may not translate into a specific researcher application running twice as fast on the new hardware. Some applications will run faster and some slower. Researchers are therefore encouraged to experiment to determine the optimal hardware to run their application on to minimise their SU charges.
For non-computational services such as storage, SU charging rates are defined based on the cost of providing that service. As new equipment is purchased, costs are projected over the expected service lifetime of additional equipment and then averaged across all the equipment for a service.
Depending on the service, the costs making up an SU may include:
- hardware replacement/lease/expansion costs
- maintenance costs
- utility costs (power & cooling)
- software/ hardware licencing
- share of data centre costs
- staffing costs
- administrative costs
- overheads for shared services
- expected utilisation
The monetary value of a SU is therefore based on the cost recovery for operation of the service.
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