  This paper studies the energy efficiency of composable datacentre (DC)
infrastructures over network topologies. Using a mixed integer linear
programming (MILP) model, we compare the performance of disaggregation at
rack-scale and pod-scale over selected electrical, optical and hybrid network
topologies relative to a traditional DC. Relative to a pod-scale DC, the
results show that physical disaggregation at rack-scale is sufficient for
optimal efficiency when the optical network topology is adopted and resource
components are allocated in a suitable manner. The optical network topology
also enables optimal energy efficiency in composable DCs. The paper also
studies logical disaggregation of traditional DC servers over an optical
network topology. Relative to physical disaggregation at rack-scale, logical
disaggregation of server resources within each rack enables marginal fall in
the total DC power consumption (TDPC) due to improved resource demands
placement. Hence, an adaptable composable infrastructure that can support both
in memory (access) latency sensitive and insensitive workloads is enabled. We
also conduct a study of the adoption of micro-service architecture in both
traditional and composable DCs. Our results show that increasing the modularity
of workloads improves the energy efficiency in traditional DCs, but
disproportionate utilization of DC resources persists. A combination of
disaggregation and micro-services achieved up to 23% reduction in the TDPC of
the traditional DC by enabling optimal resources utilization and energy
efficiencies. Finally, we propose a heuristic for energy efficient placement of
workloads in composable DCs which replicates the trends produced by the MILP
model formulated in this paper.
