identifiers not by parsing C code but by feeding carefully constructed
programs into the system C compiler and interpreting the generated
error messages, debug information, and object files. In practice,
parsing these is significantly less work and more robust than parsing
C source.

Cgo first invokes gcc -E -dM on the preamble, in order to find out
about simple #defines for constants and the like. These are recorded
for later use.

- Number of errors reported to the metric `storage_operation_duration_seconds_count` for emptyDir decreased significantly because previously one error was reported for each projected volume created. ([#117022](https://github.com/kubernetes/kubernetes/pull/117022), [@mpatlasov](https://github.com/mpatlasov)) [SIG Storage]

### Bug or Regression

- An inefficient lock in EndpointSlice controller metrics cache has been reworked. Network programming latency may be significantly reduced in certain scenarios, especially in clusters with a large number of Services. ([#107169](https://github.com/kubernetes/kubernetes/pull/107169), [@robscott](https://github.com/robscott)) [SIG Apps and Network]

- Improved exponential backoff in Reflector, significantly reducing the load on Kubernetes apiserver in case of throttling of requests. ([#118132](https://github.com/kubernetes/kubernetes/pull/118132), [@marseel](https://github.com/marseel)) [SIG API Machinery and Scalability]

- Optimized kube-proxy conntrack cleanup logic, reducing the time complexity of deleting stale UDP entries. This significantly improves performance when there are many stale connections to clean up. ([#135511](https://github.com/kubernetes/kubernetes/pull/135511), [@aojea](https://github.com/aojea)) [SIG Network]

    Potential impacts:
    If an SLO/alert has been defined based on `volume_operation_total_seconds`, it might get violated because of the more accurate metric might be significantly larger than previously reported. The metric is defined to be a histogram and the new semantic could change the distribution. ([#78061](https://github.com/kubernetes/kubernetes/pull/78061), [@yuxiangqian](https://github.com/yuxiangqian))

- Number of errors reported to the metric `storage_operation_duration_seconds_count` for emptyDir decreased significantly because previously one error was reported for each projected volume created. ([#117022](https://github.com/kubernetes/kubernetes/pull/117022), [@mpatlasov](https://github.com/mpatlasov)) [SIG Storage]

### Other notable announcements

SIG Network is moving IPv6 to Beta in Kubernetes 1.18, after incrementing significantly the test coverage with new CI jobs.

## Before Upgrading

Consider the following changes, limitations, and guidelines before you upgrade:

### **API Machinery**

* Ensure the GCI metadata files do not have newline at the end ([#38727](https://github.com/kubernetes/kubernetes/pull/38727), [@Amey-D](https://github.com/Amey-D))
* Significantly speed-up make ([#38700](https://github.com/kubernetes/kubernetes/pull/38700), [@sttts](https://github.com/sttts))