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Chapter 13. Using CPU Manager

13.1. What CPU Manager Does

CPU Manager manages groups of CPUs and constrains workloads to specific CPUs.

CPU Manager is useful for workloads that have some of these attributes:

  • Require as much CPU time as possible.
  • Are sensitive to processor cache misses.
  • Are low-latency network applications.
  • Coordinate with other processes and benefit from sharing a single processor cache.

13.2. Setting up CPU Manager

To set up CPU Manager:

  1. Optionally, label a node:

    Copy to Clipboard Toggle word wrap 
    # oc label node perf-node.example.com cpumanager=true

  2. Enable CPU manager support on the target node:

    Copy to Clipboard Toggle word wrap 
    # oc edit configmap <name> -n openshift-node

    For example:

    Copy to Clipboard Toggle word wrap 
    # oc edit cm node-config-compute -n openshift-node

    Example Output

    Copy to Clipboard Toggle word wrap 
    ...
kubeletArguments:
...
  feature-gates:
  - CPUManager=true
  cpu-manager-policy:
  - static
  cpu-manager-reconcile-period:
  - 5s
  system-reserved:
    1 
    
  - cpu=500m

    Copy to Clipboard Toggle word wrap 
    # systemctl restart atomic-openshift-node
    1
    system-reserved is a required setting. The value might need to be adjusted depending on your environment.

  3. Create a pod that requests a core or multiple cores. Both limits and requests must have their CPU value set to a whole integer. That is the number of cores that will be dedicated to this pod:

    Copy to Clipboard Toggle word wrap 
    # cat cpumanager.yaml

    Example Output

    Copy to Clipboard Toggle word wrap 
    apiVersion: v1
kind: Pod
metadata:
  generateName: cpumanager-
spec:
  containers:
  - name: cpumanager
    image: gcr.io/google_containers/pause-amd64:3.0
    resources:
      requests:
        cpu: 1
        memory: "1G"
      limits:
        cpu: 1
        memory: "1G"
  nodeSelector:
    cpumanager: "true"

  4. Create the pod:

    Copy to Clipboard Toggle word wrap 
    # oc create -f cpumanager.yaml

  5. Verify that the pod is scheduled to the node that you labeled:

    Copy to Clipboard Toggle word wrap 
    # oc describe pod cpumanager

    Example Output

    Copy to Clipboard Toggle word wrap 
    Name:         cpumanager-4gdtn
Namespace:    test
Node:         perf-node.example.com/172.31.62.105
...
    Limits:
      cpu:     1
      memory:  1G
    Requests:
      cpu:        1
      memory:     1G
...
QoS Class:       Guaranteed
Node-Selectors:  cpumanager=true
                 region=primary

  6. Verify that the cgroups are set up correctly. Get the PID of the pause process:

    Copy to Clipboard Toggle word wrap 
    # systemd-cgls -l

    Example Output

    Copy to Clipboard Toggle word wrap 
    ├─1 /usr/lib/systemd/systemd --system --deserialize 20
├─kubepods.slice
│ ├─kubepods-pod0ec1ab8b_e1c4_11e7_bb22_027b30990a24.slice
│ │ ├─docker-b24e29bc4021064057f941dc5f3538595c317d294f2c8e448b5e61a29c026d1c.scope
│ │ │ └─44216 /pause

    Pods of QoS tier Guaranteed are placed within the kubepods.slice. Pods of other QoS tiers end up in child cgroups of kubepods.

    Copy to Clipboard Toggle word wrap 
    # cd /sys/fs/cgroup/cpuset/kubepods.slice/kubepods-pod0ec1ab8b_e1c4_11e7_bb22_027b30990a24.slice/docker-b24e29bc4021064057f941dc5f3538595c317d294f2c8e448b5e61a29c026d1c.scope
# for i in `ls cpuset.cpus tasks` ; do echo -n "$i "; cat $i ; done

    Example Output

    Copy to Clipboard Toggle word wrap 
    cpuset.cpus 2
tasks 44216

  7. Check the allowed CPU list for the task:

    Copy to Clipboard Toggle word wrap 
    # grep ^Cpus_allowed_list /proc/44216/status

    Example Output

    Copy to Clipboard Toggle word wrap 
    Cpus_allowed_list:      2

  8. Verify that another pod (in this case, the pod in the burstable QoS tier) on the system can not run on the core allocated for the Guaranteed pod:

    Copy to Clipboard Toggle word wrap 
    # cat /sys/fs/cgroup/cpuset/kubepods.slice/kubepods-burstable.slice/kubepods-burstable-podbe76ff22_dead_11e7_b99e_027b30990a24.slice/docker-da621bea7569704fc39f84385a179923309ab9d832f6360cccbff102e73f9557.scope/cpuset.cpus
0-1,3
     Copy to Clipboard Toggle word wrap 
    # oc describe node perf-node.example.com

    Example Output

    Copy to Clipboard Toggle word wrap 
    ...
Capacity:
 cpu:     4
 memory:  16266720Ki
 pods:    40
Allocatable:
 cpu:     3500m
 memory:  16164320Ki
 pods:    40
---
  Namespace                  Name                      CPU Requests  CPU Limits  Memory Requests  Memory Limits
  ---------                  ----                      ------------  ----------  ---------------  -------------
  test                        cpumanager-4gdtn          1 (28%)       1 (28%)     1G (6%)          1G (6%)
  test                        cpumanager-hczts          1 (28%)       1 (28%)     1G (6%)          1G (6%)
  test                        cpumanager-r9wrq          1 (28%)       1 (28%)     1G (6%)          1G (6%)
...
Allocated resources:
  (Total limits may be over 100 percent, i.e., overcommitted.)
  CPU Requests  CPU Limits  Memory Requests  Memory Limits
  ------------  ----------  ---------------  -------------
  3 (85%)       3 (85%)     5437500k (32%)   9250M (55%)

    This VM has four CPU cores. You set system-reserved to 500 millicores, meaning half of one core is subtracted from the total capacity of the node to arrive at the Node Allocatable amount.

    You can see that Allocatable CPU is 3500 millicores. This means we can run three of our CPU manager pods since each will take one whole core. A whole core is equivalent to 1000 millicores.

    If you try to schedule a fourth pod, the system will accept the pod, but it will never be scheduled:

    Copy to Clipboard Toggle word wrap 
    # oc get pods --all-namespaces |grep test

    Example Output

    Copy to Clipboard Toggle word wrap 
    test              cpumanager-4gdtn               1/1       Running            0          8m
test              cpumanager-hczts               1/1       Running            0          8m
test              cpumanager-nb9d5               0/1       Pending            0          8m
test              cpumanager-r9wrq               1/1       Running            0          8m

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