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  1. Mesos
  2. MESOS-5741

Quota + reserved resources is overly conservative

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    Details

    • Type: Bug
    • Status: Open
    • Priority: Major
    • Resolution: Unresolved
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: None

      Description

      Consider this scenario:

      • Cluster has 10 CPUs total. 8 CPUs are reserved for role X, 2 CPUs are unreserved.
      • Role X has a quota for 4 CPUs, but has only been allocated 2 CPUs (e.g., because it has declined an offer for the other 2 CPUs). The CPUs it has been allocated come from the reserved resources, so there are 6 reserved CPUs and 2 unreserved CPUs available.
      • That means 6 CPUs should be offered as non-quota resources. However, which 6 CPUs should be offered – the 6 reserved CPUs, or 4 reserved CPUs and 2 unreserved CPUs?

      The current quota allocation logic appears to always offer the 6 reserved CPUs. This is unfortunate, because frameworks in other roles won't be able to use those resources. The reason for this behavior is:

        Resources remainingClusterResources = roleSorter->totalScalarQuantities();
        foreachkey (const string& role, activeRoles) {
          remainingClusterResources -= roleSorter->allocationScalarQuantities(role);
        }
      

      remainingClusterResources may have a role set (although dynamically reserved resources will have been converted into effectively static reservations).

        Resources unallocatedQuotaResources;
        foreachpair (const string& name, const Quota& quota, quotas) {
          // Compute the amount of quota that the role does not have allocated.
          //
          // NOTE: Revocable resources are excluded in `quotaRoleSorter`.
          // NOTE: Only scalars are considered for quota.
          Resources allocated = getQuotaRoleAllocatedResources(name);
          const Resources required = quota.info.guarantee();
          unallocatedQuotaResources += (required - allocated);
        }
      

      unallocatedQuotaResources will not have role set, per the implementation of getQuotaRoleAllocatedResources.

      remainingClusterResources -= unallocatedQuotaResources;
      

      This means that only unreserved resources will be subtracted from remainingClusterResources. At best this is sub-optimal, because it seems better to lay-away resources for role X that are already reserved for X. I don't think it will result in violating quota guarantees, though.

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            • Assignee:
              Unassigned
              Reporter:
              neilc Neil Conway
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              Dates

              • Created:
                Updated: