"Burke explained that previously the way cgroups worked was with
ratios for capacity, for example setting a 40 percent of CPU
policy. He noted that the trouble with this approach is that if,
for example, there were three virtual guests running on a system,
each controlled with a cgroup setting for one-third capacity, if
two systems were idle, the third could consume all of the systems
resources. With the new ability to specify capacities for I/O
rates, CPU cycles and the like, there is now a bounded ceiling.