ovsdb: monitor: Keep and maintain the initial change set.

Change sets in OVSDB monitor are storing all the changes that happened
between a particular transaction ID and now.  Initial change set
basically contains all the data.

On each monitor request a new initial change set is created by creating
an empty change set and adding all the database rows.  Then it is
converted into JSON reply and immediately untracked and destroyed.

This is causing significant performance issues if many clients are
requesting new monitors at the same time.  For example, that is
happening after database schema conversion, because conversion triggers
cancellation of all monitors.  After cancellation, every client sends
a new monitor request.  The server then creates a new initial change
set, sends a reply, destroys initial change set and repeats that for
each client.  On a system with 200 MB database and 500 clients,
cluster of 3 servers spends 20 minutes replying to all the clients
(200 MB x 500 = 100 GB):

  timeval|WARN|Unreasonably long 1201525ms poll interval

Of course, all the clients are already disconnected due to inactivity
at this point.  When they are re-connecting back, server accepts new
connections one at a time, so inactivity probes will not be triggered
anymore, but it still takes another 20 minutes to handle all the
incoming connections.

Let's keep the initial change set around for as long as the monitor
itself exists.  This will allow us to not construct a new change set
on each new monitor request and even utilize the JSON cache in some
cases.  All that at a relatively small maintenance cost, since we'll
need to commit changes to one extra change set on every transaction.
Measured memory usage increase due to keeping around a shallow copy
of a database is about 10%.  Measured CPU usage difference during
normal operation is negligible.

With this change it takes only 30 seconds to send out all the monitor
replies in the example above.  So, it's a 40x performance improvement.
On a more reasonable setup with 250 nodes, the process takes up to
8-10 seconds instead of 4-5 minutes.

Conditional monitoring will benefit from this change as well, however
results might be less impressive due to lack of JSON cache.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>

ovsdb/monitor.c

@@ -609,7 +609,10 @@ ovsdb_monitor_untrack_change_set(struct ovsdb_monitor *dbmon,
     ovs_assert(mcs);
     if (--mcs->n_refs == 0) {
         if (mcs == dbmon->init_change_set) {
-            dbmon->init_change_set = NULL;
+            /* The initial change set should exist as long as the
+             * monitor itself. */
+            mcs->n_refs++;
+            return;
         } else if (mcs == dbmon->new_change_set) {
             dbmon->new_change_set = NULL;
         }