The BtrPlace Constraints Catalog

MaxSpareResources

Definition

Signature

maxSpareResources(s : set<server>, rc : string, n : number)

s : a non-empty set of servers for a meaningful constraint.
rc : a resource identifier such as mem, ucpu, pcpu or nodes to identify the physical memory, the computational capacity, the physical CPUs or the node itself, respectively.
n : a positive number

The maxSpareResources restricts to at most n, the number of free resources directly available for VMs on the online servers in s. Servers in the Offline state are ignored.

Classification

Primary users: datacenter administrator
Manipulated elements: Resource allocation
Concerns: Resource management, Power saving, Capacity planning

Usage

This constraint deserves a power saving concern. The most effective solution to reduce the energy consumption of a non-saturated datacenter is to turn off unused servers and to turn on and off servers depending on the load variation. When a load spike arise, it may then be a necessary to put some servers online to make their resources available to the VMs. [26], [48] The time to boot the awaited servers may however be significant and alter the reactivity of the datacenter when it faces an emergency situations. One solution is to let online a controlled number of spare resources that can be used instantly to absorb the load increase. A datacenter administrator may then use one maxSpareResources constraint to control the maximum number of unused servers to let online while the others will be turned off to save power.

It is worth noting that despite this constraint is applicable to a various number of resources, it is preferable to only focus the nodes resource. Indeed, a server provides resources at a coarse grain and it may not be possible to manage the resources according to the constraint by only managing the servers state.

Example

Figure 20 depicts a sample reconfiguration between a source and a destination configuration where each server provides 8 unit of CPU and 7 unit of memory resources to VMs. During the reconfiguration, several relocations have been performed and the server N3 has been turned off to save power. In this setting, the following maxSpareResources constraints were considered:

figure 20: A reconfiguration motivated by maxSpareResources constraints.

maxSpareResources({N2,N3,N4},"node",1). This constraint was satisfied in the source configuration as no server was idle. The constraint is still satisfied in the destination configuration as only N4 is in the Online state and idle.
maxSpareResources({N1,N2,N3},"ucpu",10). This constraint was not satisfied in the source configuration as 11 uCPU was directly available to the running VMs. The reconfiguration fixed this violation. With the shutdown of N3, 8 uCPU resources are directly available in the destination configuration, which is an amount allowed by the constraint.
maxSpareResources({N1, N3},"mem",3). This constraint was not satisfied in the source configuration as 10 unit of memory were directly available to the running VMs. The reconfiguration fixed this violation. With the shutdown of N3, and the saturation of N1, no memory resources are left available to the VMs on these servers.