Add SIG Scalability FAQ

Author: wojtek-t

sig-scalability/configs-and-limits/faq.md

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+# FAQ
+
+### How many QPS kube-apiserver can support?
+
+There is NO single answer to this question. The reason for it is that,
+every Kubernetes request is potentially very different. As a result,
+kube-apiserver may be able to process N in a given usage pattern, but
+may not be able to process even N/5 in a different usage pattern.
+
+To be more specific, the difference is fairly obvious if we compare
+GET and LIST requests (e.g. "GET my-pod" vs "LIST all pods").
+It's also not counter-intuitive that there is a difference between
+ready-only and mutating requests (e.g. "GET my-pod" vs "POST my-pod").
+
+However, even if we focus only on a single type of request (let's say just
+POST or just PUT), the differences can be in the orders of magnitude.
+The two main reasons to think about are:
+- size of the object - there is a huge difference between a small Lease
+  object (say ~300B) and large custom CRD of e.g. 1MB
+- fan-out - if a given object is being watched by N watchers, you
+  suddenly get a multiplication factor for the cost of your request,
+  that as a sender of a single API call you don't fully control
+
+As a result, we are consciously not providing any values, because
+one can imagine a cluster that handles thousands of QPS and a cluster
+that is set up the same way may have troubles handling small tens of
+QPS if the requests are fairly expensive.