[ovs-discuss] Megaflow Inspection

Ben Pfaff blp at ovn.org
Tue Jan 7 22:39:20 UTC 2020

On Tue, Jan 07, 2020 at 10:44:57PM +0200, Matan Rosenberg wrote:
> Acutally, I do think I have a megaflow (or other caching) issue.
> We use OVS for L2 packet forwarding; that is, given a packet, we don't need
> OVS to look at other protocols beyond the Ethernet layer.
> Additionally, we use VXLAN to establish L2 overlay networks across multiple
> OVS servers.
> Just to make thing clear, these are some typical flow rules that you might
> see on a bridge:
> - in_port=1,actions=2,3
> - in_port=42,actions=FLOOD
> - actions=NORMAL
> No IP matching, conntrack, etc.
> We're experiencing severe performance issues with OVS - in this use case,
> it cannot handle more than couple thousand packets/s.
> After some exploring, I've noticed that the installed megaflows try to
> match on fields that are not present in the rules, apparently for no reason.
> Here's a complete example to reproduce, using OVS 2.12.0:
> # ip link add dev a-blue type veth peer name a-red
> # ip link add dev b-blue type veth peer name b-red
> # ovs-vsctl add-br br0
> # ovs-vsctl add-port br0 a-blue
> # ovs-vsctl add-port br0 b-blue
> # ovs-ofctl del-flows br0
> # ovs-ofctl add-flow br0 in_port=a-blue,actions=b-blue
> # ovs-ofctl add-flow br0 in_port=b-blue,actions=a-blue
> After injecting ~100 random packets (IP, IPv6, TCP, UDP, ARP with random
> addresses) to one of the red interfaces (with https://pastebin.com/Y6dPFCKJ),
> these are the installed flows:
> # ovs-dpctl dump-flows
> recirc_id(0),in_port(2),eth(),eth_type(0x0806), packets:54, bytes:2268,
> used:1.337s, actions:3
> recirc_id(0),in_port(2),eth(),eth_type(0x86dd),ipv6(frag=no), packets:28,
> bytes:1684, used:1.430s, flags:S, actions:3
> recirc_id(0),in_port(2),eth(),eth_type(0x0800),ipv4(frag=no), packets:15,
> bytes:610, used:1.270s, flags:S, actions:3
> As you can see, for some reason, OVS had split the single relevant OpenFlow
> rule to three separate megaflows, one for each eth_type (and even other
> fields - IP fragmentation?).
> In my production scenario, the packets are even more diversified, and we
> see OVS installing flows which match on even more fields, including
> specific Ethernet and IP addresses.
> This leads to a large number of flows that have extremely low hit rate -
> each flow handles not more than ~100 packets (!) during its entire lifetime.
> We suspect that this causes the performance peanalty; either
> 1) The EMC/megaflow table is full, so vswitchd upcalls are all over the
> place, or
> 2) The huge number of inefficient megaflows leads to terrible lookup times
> in the in-kernel megaflow table itslef (due to large number of masks, etc.)
> In short: how can I just make OVS oblivious to these fields? Why does it
> try to match on irrlevant fields?

I can see how this would be distressing.

You can use ofproto/trace with a few examples to help figure out why OVS
is matching on more fields than you expect.

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