  In hosting environments such as IaaS clouds, desirable application
performance is usually guaranteed through the use of Service Level Agreements
(SLAs), which specify minimal fractions of resource capacities that must be
allocated for use for proper operation. Arbitrary colocation of applications
with different SLAs on a single host may result in inefficient utilization of
the host's resources. In this paper, we propose that periodic resource
allocation and consumption models be used for a more granular expression of
SLAs. Our proposed SLA model has the salient feature that it exposes
flexibilities that enable the IaaS provider to safelya transform SLAs from one
form to another for the purpose of achieving more efficient colocation. Towards
that goal, we present MorphoSys: a framework for a service that allows the
manipulation of SLAs to enable efficient colocation of workloads. We present
results from extensive trace-driven simulations of colocated Video-on-Demand
servers in a cloud setting. The results show that potentially-significant
reduction in wasted resources (by as much as 60%) are possible using MorphoSys.
