Add short segment on VMs / OpenVZ

doc/sphinx/tutorial/index.rst

@@ -26,6 +26,7 @@ separate topic. Good luck.
 	vary
 	purging
 	esi
+	virtualised
 	advanced_backend_servers
         handling_misbehaving_servers
         advanced_topics

doc/sphinx/tutorial/virtualised.rst

+
+Running inside a virtual machine (VM)
+-------------------------------------
+
+It is possible, but not recommended for high performance, to run Varnish on virtualised 
+hardware.
+
+OpenVZ
+''''''
+
+If you are running on 64bit OpenVZ (or Parallels VPS), you must reduce the 
+maximum stack size before starting Varnish. The default allocates to much memory per thread, 
+which will make varnish fail as soon as the number of threads (==traffic) increases.
+
+Reduce the maximum stack size by running::
+
+    ulimit -s 256
+
+in the startup script.
+