  Grid computing typically provides most of the data processing resources for
large High Energy Physics experiments. However typical grid sites are not fully
utilized by regular workloads. In order to increase the CPU utilization of
these grid sites, the ATLAS@Home volunteer computing framework can be used as a
backfilling mechanism. Results show an extra 15% to 42% of CPU cycles can be
exploited by backfilling grid sites running regular workloads while the overall
CPU utilization can remain over 90%. Backfilling has no impact on the failure
rate of the grid jobs, and the impact on the CPU efficiency of grid jobs varies
from 1% to 11% depending on the configuration of the site. In addition the
throughput of backfill jobs in terms of CPU time per simulated event is the
same as for resources dedicated to ATLAS@Home. This approach is sufficiently
generic that it can easily be extended to other clusters.
