[ovs-dev] [OVN][Scale] Conjunctive matches exponentially increase in Table 45

Krzysztof Klimonda kklimonda at syntaxhighlighted.com
Tue Mar 16 00:17:37 UTC 2021


Hi,

Sorry for what is most likely an unconnected reply to a thread - I can't seem to figure out how to reply to a thread from before I was subscribed to ML.

We've been testing OVN scaling for our OpenStack cloud, and found what seems to be a OF flow explosion that is basically a mirror of the issue reported by Girish a week ago or so.

In OpenStack, neutron creates a "default" security group that has 4 rules (2 for both IPv4 and IPv6):

- allow all egress traffic from the port
- allow all ingress traffic from other ports belonging to the same default group

What we have discovered in our testing, is that this second rule translates into the following ACL in OVN:

```
outport == @pg_304cc336_8db3_4efd_a558_408e648e6259 && ip4 && ip4.src == $pg_304cc336_8db3_4efd_a558_408e648e6259_ip4
```
where port_group `pg_304cc336_8db3_4efd_a558_408e648e6259_ip4` is defined in nbdb and contains all ports attached to the SG, and address_set pg_304cc336_8db3_4efd_a558_408e648e6259_ip4 is defined in sbdb and seems to have a list of addresses that are assigned to ports from that port_group[1].

As Girish has explained in his email, such ACLs are translated into a bunch of duplicated flows that only seem to differ in metadata:

```
# ovs-ofctl dump-flows br-int |egrep "(12474|12475)"
[...]
 cookie=0x0, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,ip,reg0=0x100/0x100,reg15=0x3,metadata=0x20e actions=conjunction(12475,2/2)
 cookie=0x0, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,ip,reg0=0x100/0x100,metadata=0x20e,nw_src=1.0.0.67 actions=conjunction(12475,1/2)
 cookie=0x0, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,ip,reg0=0x100/0x100,metadata=0x20e,nw_src=2.0.0.52 actions=conjunction(12475,1/2)
 cookie=0x0, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,ip,reg0=0x100/0x100,metadata=0x20d,nw_src=1.0.0.67 actions=conjunction(12475,1/2)
 cookie=0x0, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,ip,reg0=0x100/0x100,metadata=0x20d,nw_src=2.0.0.52 actions=conjunction(12475,1/2)
 cookie=0xb25108c3, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,conj_id=12475,ip,reg0=0x100/0x100,metadata=0x20e actions=resubmit(,46)
 cookie=0xb25108c3, duration=47132.116s, table=45, n_packets=0, n_bytes=0, idle_age=47132, priority=2002,conj_id=12475,ip,reg0=0x100/0x100,metadata=0x20d actions=resubmit(,46)
[...]
#
```
(See http://paste.openstack.org/show/803598/ for the full output of grep)

His idea of changing this conjunction into one that matches additionally on metadata seems to make sense in this particular instance, given that all ports from all datapaths need to evaluate same set of rules, and possibly it makes sense for all ACLs too?

Anyway, to understand how OF flows are generated by ovn-controller, I took a quick look at the source code, and it seems that right now all flows are forcefully matched to their datapath (by unconditional matching on metadata field).
Would it make sense to introduce a notion of "datapath unbound flow" when conjunction is already matching metadata?
Are there some other parts of OVN code that heavily depend on flows being installed per-dp?
How would that affect OVS performance when matching packets in userspace? In our testing we've ended up with over 1M flows installed in table 45, which seems to be dwarfing any potential performance loss from having flows that don't match on metadata field, but perhaps I'm wrong? Still, that's a lot of flows, and puts a hard scaling limit on some openstack deployments given it's a SG that is by default attached to all ports on all VMs.


[1] (although apparently not additional IP addresses allowed on port via allowed-address-pair - I think I've seen this issue before while testing magnum.


-- 
  Krzysztof Klimonda
  kklimonda at syntaxhighlighted.com


More information about the dev mailing list