docs: Add section about capacity planning

docs/Guides/Recommendations.md

@@ -8,7 +8,8 @@ This document contains a set of recommendations when using Fastify.
   - [HAProxy](#haproxy)
   - [Nginx](#nginx)
 - [Kubernetes](#kubernetes)
-
+- [Kubernetes](#kubernetes)
+- [Capacity Planning For Production](#capacity)
 ## Use A Reverse Proxy
 <a id="reverseproxy"></a>
 
@@ -298,3 +299,38 @@ readinessProbe:
     timeoutSeconds: 3
     successThreshold: 1
     failureThreshold: 5
+```
+
+## Capacity Planning For Production
+<a id="capacity"></a>
+
+In order to rightsize the production environment for your Fastify application, 
+you are highly recommended to perform your own measurements against 
+different configurations of the environment, which may
+use real CPU cores, virtual CPU cores (vCPU), or even fractional
+vCPU cores. We will use the term vCPU throughout this
+recommendation to represent any CPU type.
+
+You can use such tools as [k6](https://github.com/grafana/k6) 
+or [autocannon](https://github.com/mcollina/autocannon) for conducting
+the necessary performance tests.
+
+That said, you may also consider the following as a rule of a thumb:
+
+* In order to have the lowest possible latency, 2 vCPU are recommended per app 
+instance (e.g., a k8s pod). The second vCPU will mostly be used by the 
+garbage collector (GC) and libuv threadpool. This will minimize the latency 
+for your users, as well as the memory usage, as the GC will be run more
+frequently. Also, the main thread won't have to stop to let the GC run.
+
+* In order to optimize for throughput (handling the largest possible amount of
+requests per second per vCPU available), consider using smaller amount of vCPUs
+per app instance. It is totally fine to run Node.js application with 1 vCPU.
+
+* You may experiment with even smaller amount of vCPU, which may provide even 
+better throughput in certain use-cases. There are reports of e. g. API gateway
+solutions working well with 100m-200m vCPU in Kubernetes.
+
+See [Node's Event Loop From the Inside Out ](https://www.youtube.com/watch?v=P9csgxBgaZ8)
+in order to understand the workings of Node.js in greater detail and make a
+better determination about what your specific application needs.