ovsdb-idl: Re-parse backrefs of inserted rows only once.

While adding new rows ovsdb-idl re-parses all the other rows that
references this new one.  For example, current ovn-kubernetes creates
load balancers and adds the same load balancer to all logical switches
and logical routers.  So, then a new load balancer is added, rows for
all logical switches and routers re-parsed.

During initial database connection (or re-connection with
monitor/monitor_cond or monitor_cond_since with outdated last
transaction id) the client downloads the whole content of a database.
In case of OVN, there might be already thousands of load balancers
configured.  ovsdb-idl will process rows in that initial monitor reply
one-by-one.  Therefore, for each load balancer row, it will re-parse
all rows for switches and routers.

Assuming that we have 120 Logical Switches and 30K load balancers.
Processing of the initial monitor reply will take 120 (switch rows) *
30K (load balancer references in a switch row) * 30K (load balancer
rows) = 10^11 operations, which may take hours.  ovn-kubernetes will
use LB groups eventually, but there are other less obvious cases that
cannot be changed that easily.

Re-parsing doesn't change any internal structures of the IDL.  It
destroys and re-creates exactly same arcs between rows.  The only
thing that changes is the application-facing array of pointers.

Since internal structures remains intact, suggested solution is to
postpone the re-parsing of back references until all the monitor
updates processed.  This way we can re-parse each row only once.

Tested in a sandbox with 120 LSs, 120 LRs and 3K LBs, where each
load balancer added to each LS and LR, by re-statring ovn-northd and
measuring the time spent in ovsdb_idl_run().

Before the change:

  OVN_Southbound: ovsdb_idl_run took: 924 ms
  OVN_Northbound: ovsdb_idl_run took: 825118 ms  --> 13.75 minutes!

After:

  OVN_Southbound: ovsdb_idl_run took: 692 ms
  OVN_Northbound: ovsdb_idl_run took: 1698 ms

Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

0 files changed, 0 insertions, 0 deletions