[ovs-discuss] ovs-vswitchd port limit with OpenStack

Flavio Leitner fbl at sysclose.org
Thu May 2 19:44:42 UTC 2019

On Tue, Apr 30, 2019 at 04:50:48PM -0700, Ben Pfaff wrote:
> On Fri, Apr 26, 2019 at 11:52:22AM -0500, William Konitzer wrote:
> > I'm reading
> > (http://www.openvswitch.org/support/dist-docs/ovs-vswitchd.8.txt
> > section LIMITS) and it says "Performance will degrade beyond 1,024
> > ports per bridge due to fixed hash table sizing.” Do we have a little
> > more info on what that means and what to expect for less experienced
> > users like myself?
> I think that this comment is now obsolete.  There was a fairly recent
> change that should have reduced the cost of a port.  The kernel hash
> table is still fixed in size but I don't think it's accessed on any fast
> path so I think in practice it doesn't matter.
> > The background here is we’re working with OpenStack and seeing
> > performance issues when lots of networks are created.. Once we have
> > more than about 1500 ports on the br-int on a gateway node it seems to
> > take a long time to add new ports.

You might want to bump the default netdev_max_backlog because that
is the maximum amount of packets queued. So, if you have too many
ports, there will be either packet loss, or slow path'ed traffic.


> > 
> > I’m trying to quickly determine if we have a config issue, an
> > Openstack issue or whether we’re hitting some sort of OVS limit as
> > described. It seems to me that 1500 ports isn’t that many, but I’m not
> > sure what sort of performance degradation I should be expecting above
> > 1024 ports. The gateway node is so lightly loaded that I’d prefer to
> > be able to handle a lot more networks on it before deploying another
> > one.
> Are you adding ports one at a time with ovs-vsctl?  If you can add them
> in a batch, it will perform better.  I guess we could also add a "daemon
> mode" like ovn-nbctl, which would help a good deal too.
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