lib: Add some documentation to wheel.h

Add some hopefully useful documentation to the
timer wheel.h code.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

lib/wheel.h

@@ -40,9 +40,49 @@ struct timer_wheel
   void (*slot_run) (void *);
 };
 
+/*
+ * Creates a timer wheel
+ *
+ * master - Thread master structure for the process
+ * period - The Time in seconds that the timer wheel will
+ *          take before it starts issuing commands again
+ *          for items in each slot
+ * slots  - The number of slots to have in this particular
+ *          timer wheel
+ * slot_key - A hashing function of some sort that will allow
+ *            the timer wheel to put items into individual slots
+ * slot_run - The function to run over each item in a particular slot
+ *
+ * Creates a timer wheel that will wake up 'slots' times over the entire
+ * wheel.  Each time the timer wheel wakes up it will iterate through
+ * and run the slot_run function for each item stored in that particular
+ * slot.
+ *
+ * The timer code is 'intelligent' in that it notices if anything is
+ * in a particular slot and can schedule the next timer to skip
+ * the empty slot.
+ *
+ * The general purpose of a timer wheel is to reduce events in a system.
+ * A perfect example of usage for this is say hello packets that need
+ * to be sent out to all your neighbors.  Suppose a large routing protocol
+ * has to send keepalive packets every Y seconds to each of it's peers.
+ * At scale we can have a very large number of peers, X.
+ * This means that we will have X timing events every Y seconds.
+ * If you replace these events with a timer wheel that has Z slots
+ * you will have at most Y/Z timer events if each slot has a work item
+ * in it.
+ *
+ * When X is large the number of events in a system can quickly escalate
+ * and cause significant amount of time handling thread events instead
+ * of running your code.
+ */
 struct timer_wheel *wheel_init (struct thread_master *master, int period, size_t slots,
 				unsigned int (*slot_key) (void *),
 				void (*slot_run) (void *));
+
+/*
+ * Delete the specified timer wheel created
+ */
 void wheel_delete (struct timer_wheel *);
 
 /*
@@ -51,17 +91,25 @@ void wheel_delete (struct timer_wheel *);
 int wheel_stop (struct timer_wheel *wheel);
 
 /*
- * Start the wheel from running again
+ * Start the wheel running again
  */
 int wheel_start (struct timer_wheel *wheel);
 
 /*
+ * wheel - The Timer wheel being modified
+ * item - The generic data structure that will be handed
+ *        to the slot_run function.
+ *
  * Add item to a slot setup by the slot_key,
  * possibly change next time pop.
  */
 int wheel_add_item (struct timer_wheel *wheel, void *item);
 
 /*
+ * wheel - The Timer wheel being modified.
+ * item - The item to remove from one of the slots in
+ *        the timer wheel.
+ *
  * Remove a item to a slot setup by the slot_key,
  * possibly change next time pop.
  */