update documentation on how to set ulimit for Redis (#508)

* update documentation on how to set ulimit for Redis

* Move documentation to troubleshooting section.

* Add another section header.

* update redis file descriptor docs: simplify command

doc/source/using-ray-on-a-large-cluster.md

@@ -237,6 +237,8 @@ Note that the destination argument for this command must represent an absolute p
 
 ## Troubleshooting
 
+### Problems with parallel-ssh
+
 If any of the above commands fail, verify that the head node has SSH access to
 the other nodes by running
 
@@ -253,3 +255,19 @@ node with agent forwarding enabled. This is done as follows.
 ssh-add <ssh-key>
 ssh -A ubuntu@<head-node-public-ip>
 ```
+
+### Configuring EC2 instances to increase the number of allowed Redis clients
+
+This section can be ignored unless you run into problems with the maximum
+number of Redis clients.
+
+* Ensure that the hard limit for the number of open file descriptors is set
+  to a large number (e.g., 65536). This only needs to be done on instances
+  where Redis shards will run --- by default, just the _head node_.
+  - Check the hard ulimit for open file descriptors with `ulimit -Hn`
+  - If that number is smaller than 65536, set the hard ulimit for open file descriptors
+    system-wide:
+    ```
+    sudo bash -c "echo $USER hard nofile 65536 >> /etc/security/limits.conf"
+    ```
+  - Logout and log back in