container/libcontainer: Improve limits file parsing perf #3396
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Only max open files is ever parsed from limits files, therefore this
change optimizes for that case.
Benchmark:
On a GKE v1.27.4 production cluster, this code path used roughly 1.5% of
the total kubelet CPU usage, and at 98.44% improvement this likely
results in at least a 1.5% CPU reduction (perhaps even more since also
less garbage is produced to be collected by the GC).
This change came out of a live stream that we do on a weekly basis trying to improve performance of popular open source software. Episode is here: https://www.youtube.com/watch?v=ricEoAdJNHc