Remove force sync for non-external backends

[ZxYuan]

In PR #5726 and #5418, dynamic LB sync was applied to prevent syncing per second for non-external backends. However, a strategy of force syncing non-external backends per 30s was also added. Practically, the additional strategy is unnecessary but leads to waste of CPUs when number of ingresses/services is large.

What this PR does / why we need it:

This PR remove force sync for non-external backends.
The figures below show how much impact current LB has on CPU usage (%) with/without FORCE sync (No input traffic into nginx). Without FORCE sync, CPU usage per nginx worker keeps <1%.
CPU model name: Intel(R) Xeon(R) Silver 4208 CPU @ 2.10GHz

1000 services&ingresses with FORCE sync

2000 services&ingresses with FORCE sync

3000 services&ingresses with FORCE sync

4000 services&ingresses with FORCE sync

5000 services&ingresses with FORCE sync

6000 services&ingresses with FORCE sync

7000 services&ingresses with FORCE sync

8000 services&ingresses with FORCE sync

8000 services&ingresses without FORCE sync

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

How Has This Been Tested?

Create a nginx deployment on port 80 with label app: nginx.
Save the following yaml as press.yaml.tmpl.

kind: Service
apiVersion: v1
metadata:
  name: service-press-HAHA
spec:
  selector:
    app: nginx
  ports:
    - protocol: TCP
      port: 80
---
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: ingress-press-HAHA
spec:
  rules:
  - http:
      paths:
      - path: /press/HAHA
        backend:
          serviceName: service-press-HAHA
          servicePort: 80

Run the bash script

for i in {1..7000}
do
    sed s#HAHA#${i}#g press.yaml.tmpl > tmp.yaml
    kubectl apply -f tmp.yaml
done

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

@ZxYuan: This issue is currently awaiting triage.

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[k8s-ci-robot]

Hi @ZxYuan. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[rikatz]

/ok-to-test
/cc @tao12345666333 @moonming @ElvinEfendi

[rikatz]

/assign @tao12345666333

[rikatz]

/lgtm
/approve
Thanks

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rikatz, ZxYuan

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