Added introduction and PostgreSQL warning.

matrix/synapse/workers.md

@@ -3,7 +3,7 @@ gitea: none
 include_toc: true
 ---
 
-# Worker-based setup
+# Introduction to a worker-based setup
 
 Very busy servers are brought down because a single thread can't keep up with
 the load. So you want to create several threads for different types of work.
@@ -22,6 +22,27 @@ freeze the rest.
 
 Workers communicate with each other via socket files and Redis.
 
+**Important note**
+
+While the use of workers can drastically improve speed, the law of diminished
+returns applies. Splitting off more and more workers will not further improve
+speed after a certain point. Plus: you need to understand what the most
+resource-consuming tasks are before you can start to plan how many workers for
+what tasks you need.
+
+In this document we'll basically create a worker for every task, and several
+workers for a few heavy tasks, as an example. You mileage may not only vary, it
+will.
+
+Tuning the rest of the machine and network also counts, especially PostgreSQL.
+A well-tuned PostgreSQL can make a really big difference and should probably
+be considered even before configuring workers.
+
+With workers, PostgreSQL's configuration should be changed accordingly: see
+[Tuning PostgreSQL for a Matrix Synapse
+server](https://tcpipuk.github.io/postgres/tuning/index.html) for hints and
+examples.
+
 
 # Redis