First draft to describe how to size the native memory used by Beats i…

…nput to decode a batch of events.

docs/index.asciidoc

@@ -100,6 +100,28 @@ Setting direct memory too low decreases the performance of ingestion.
 
 NOTE: Be sure that heap and direct memory combined does not exceed the total memory available on the server to avoid an OutOfDirectMemoryError
 
+[id="plugins-{type}s-{plugin}-memory-sizing"]
+===== How to size the direct memory used
+
+To correctly size the direct memory to sustain the flow of incoming Beats connections, the medium size of the transmitted
+log lines has to be known and also the batch size used by Beats (default to 2048). Overall the connections, only a
+subset of them are actively processed in parallel by Netty, corresponding to the number of workers which equals the
+number of CPU cores available. For each under processing channel a batch of events is read and due to the way
+the decompressing and decoding part works, it keeps two copies of the batch in memory.
+The expression used to calculate the maximum direct memory usage is:
+["source","text"]
+-----
+event size * batch size * 2 * netty workers
+-----
+
+Supposing a 1Kb event size, there a small overhead of ~500 bytes of metadata transferred, considering 12 core CPU the memory
+consumption could be estimated as:
+["source","text"]
+-----
+1,5 KB * 2048 * 2 * 12
+-----
+This totalling to about 140MB. So if you have some data about the medium size of the events to process you can size
+the memory accordingly without risking to go in Out-Of-Memory error on the direct memory space in production environment.
 
 //Content for Beats
 ifeval::["{plugin}"=="beats"]