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cnf-configuring-fifo-priority-scheduling-for-ptp.adoc

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Configuring FIFO priority scheduling for PTP hardware

In telco or other deployment types that require low latency performance, PTP daemon threads run in a constrained CPU footprint alongside the rest of the infrastructure components. By default, PTP threads run with the SCHED_OTHER policy. Under high load, these threads might not get the scheduling latency they require for error-free operation.

To mitigate against potential scheduling latency errors, you can configure the PTP Operator linuxptp services to allow threads to run with a SCHED_FIFO policy. If SCHED_FIFO is set for a PtpConfig CR, then ptp4l and phc2sys will run in the parent container under chrt with a priority set by the ptpSchedulingPriority field of the PtpConfig CR.

Note

Setting ptpSchedulingPolicy is optional, and is only required if you are experiencing latency errors.
Procedure

  1. Edit the PtpConfig CR profile:

    $ oc edit PtpConfig -n openshift-ptp

  2. Change the ptpSchedulingPolicy and ptpSchedulingPriority fields:

    apiVersion: ptp.openshift.io/v1
kind: PtpConfig
metadata:
  name: <ptp_config_name>
  namespace: openshift-ptp
...
spec:
  profile:
  - name: "profile1"
...
    ptpSchedulingPolicy: SCHED_FIFO (1)
    ptpSchedulingPriority: 10 (2)

    1. Scheduling policy for ptp4l and phc2sys processes. Use SCHED_FIFO on systems that support FIFO scheduling.

    2. Required. Sets the integer value 1-65 used to configure FIFO priority for ptp4l and phc2sys processes.

  3. Save and exit to apply the changes to the PtpConfig CR.

Verification

  1. Get the name of the linuxptp-daemon pod and corresponding node where the PtpConfig CR has been applied:

    $ oc get pods -n openshift-ptp -o wide
    Example output
    NAME                            READY   STATUS    RESTARTS   AGE     IP            NODE
linuxptp-daemon-gmv2n           3/3     Running   0          1d17h   10.1.196.24   compute-0.example.com
linuxptp-daemon-lgm55           3/3     Running   0          1d17h   10.1.196.25   compute-1.example.com
ptp-operator-3r4dcvf7f4-zndk7   1/1     Running   0          1d7h    10.129.0.61   control-plane-1.example.com

  2. Check that the ptp4l process is running with the updated chrt FIFO priority:

    $ oc -n openshift-ptp logs linuxptp-daemon-lgm55 -c linuxptp-daemon-container|grep chrt
    Example output
    I1216 19:24:57.091872 1600715 daemon.go:285] /bin/chrt -f 65 /usr/sbin/ptp4l -f /var/run/ptp4l.0.config -2  --summary_interval -4 -m