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

Flavio Leitner fbl at sysclose.org
Thu May 2 19:49:17 UTC 2019

On Thu, May 02, 2019 at 04:44:42PM -0300, Flavio Leitner via discuss wrote:
> 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.

To clarify, it depends on the actions. If you are using action
NORMAL and there is a broadcast for example, all ports need a
packet copy, which means more than 1k packets will be queued.
IIRC OvS will slow path this case to prevent packet loss in
the recent versions.


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