[Perf] Optimize findMigWithMatchingBasename with O(1) prefix map lookup

[tetianakh]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Implements O(1) prefix map lookup in findMigWithMatchingBasename to avoid O(N) linear search over all MIGs when resolving MIG for an instance.

Whn GetMigforInstance is called for deleted nodes, the deleted nodes are never in the cache, so findMigWithMatchingBasename is always executed. On a large cluster that has a lot of deleted nodes, each GetMigforInstance call site (which iterates over all nodes) can take a few seconds.

Benchmark results (scale: 200 MIGs, 200 ready nodes, 100 deleted nodes, 100 upcoming nodes):

Metric	Before (Unoptimized)	After (Optimized)	Difference
Time	5.099 ms ± 27%	3.230 ms ± 40%	-36.64%
Memory	1164.6 KiB ± 1%	563.9 KiB ± 1%	-51.58%
Allocs	3.398k ± 2%	8.246k ± 0%	+142.69%

The optimization reduces CPU time by ~36% and memory allocations by ~50% at this scale.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

Hi @tetianakh. 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.

Tip

We noticed you've done this a few times! Consider joining the org to skip this step and gain /lgtm and other bot rights. We recommend asking approvers on your previous PRs to sponsor you.

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.

Details

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-sigs/prow repository.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tetianakh
Once this PR has been reviewed and has the lgtm label, please assign feiskyer for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• cluster-autoscaler/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment