Resources may get over allocated to some roles while fail to meet the quota of other roles.

In the quota role allocation stage, if a role gets some resources on an agent to meet its quota, it will also get all other resources on the same agent that it does not have quota for. This may starve roles behind it that have quotas set for those resources.

To fix that, we need to track quota headroom in the quota role allocation stage. In that stage, if a role has no quota set for a scalar resource, it will get that resource only when two conditions are both met:

• It got some other resources on the same agent to meet its quota; And

• After allocating those resources, quota headroom is still above the required amount.