Thinner Clouds with Preallocation

Different companies sharing the same cloud infrastructure often prefer to run their virtual machines (VMs) in isolation, i.e., one VM per physical machine (PM) core, due to security and efficiency concerns. To accommodate load spikes, e.g., those caused by flash-crowds, each service is allocated more machines than necessary for its instantaneous load. However, flash-crowds of different hosted services are not correlated, so at any given time, only a subset of the machines are used.

We present here the concept of preallocation—having a single physical machine ready to quickly run one of a few possible VMs, without ever running more than one at a given time. The preallocated VMs are initialized and then paused by the hypervisor. We suggest a greedy preallocation strategy, and evaluate it by simulation, using workloads based on previous analyses of flash-crowds. We observe a reduction of 35-50% in number of PMs used compared with classical dynamic allocation. This means that a datacenter can provide per-service isolation with 35%-50% fewer PMs.