[enterprise-3.9] Optimizations for HAProxy reloads

(cherry picked from commit 253c88d) xref:openshift#6744

Author: jmencak

scaling_performance/routing_optimization.adoc

@@ -125,3 +125,18 @@ accepted by load balancers provided by many public cloud providers. However,
 this affects the total memory use, especially when large numbers of connections
 are open. With very large numbers of open connections, the memory usage will be
 nearly proportionate to the increase of this tunable parameter.
+
+[[optimizations-for-haproxy-reloads]]
+==== Optimizations for HAProxy Reloads
+
+Long-lasting connections, such as WebSocket connections, combined with
+long client/server HAProxy timeouts and short HAProxy
+reload intervals, can cause instantiation of many HAProxy processes.
+These processes must handle old connections, which were started
+before the HAProxy configuration reload. A large number of these processes is
+undesirable, as it will exert unnecessary load on the system and can
+lead to issues, such as out of memory conditions.
+
+xref:../architecture/topics/router_environment_variables.adoc#[Router environment variables] affecting this
+behavior are `ROUTER_DEFAULT_TUNNEL_TIMEOUT`, `ROUTER_DEFAULT_CLIENT_TIMEOUT`, 
+`ROUTER_DEFAULT_SERVER_TIMEOUT`, and `RELOAD_INTERVAL` in particular.