On Thu, Oct 09, 2014 at 10:14:36AM +0900, Simon Horman wrote: > On Fri, Sep 26, 2014 at 04:57:25PM -0700, Ben Pfaff wrote: > > On Thu, Sep 18, 2014 at 10:55:10AM +0900, Simon Horman wrote: > > > This patch is a prototype and has several limitations: > > > > > > * It assumes that no actions follow a select group action > > > because the resulting packet after a select group action may > > > differ depending on the bucket used. It may be possible > > > to address this problem using recirculation. Or to not use > > > the datapath select group in such situations. In any case > > > this patch does not solve this problem or even prevent it > > > from occurring. > > > > It seems like this limitation in particular is a pretty big one. Do > > you have a good plan in mind for how to resolve it? > > Hi Ben, > > it seems to me that this would be somewhat difficult to resolve in the > datapath so I propose not doing so. And I have two ideas on how to > resolve this problem outside of the datapath. > > 1. Recirculation > > It seems to me that it ought to be possible to handle this by > recirculating if actions occur after an ODP select group action. > > This could be made slightly more selective by only recirculating > if the execution different buckets may result in different packet > contents and the actions after the ODP select group action rely on > the packet contents (e.g. set actions do but output actions do not). > > My feeling is that this could be implemented by adding a small amount > of extra state to action translation in ovs-vswitchd. > > 2. Fall back to selecting buckets in ovs-vswtichd > > The idea here is to detect cases where there would be a problem > executing actions after an ODP select group action and in that > case to select buckets in ovs-vswtichd: that is use the existing bucket > translation code in ovs-vswtichd. > > Though this seems conceptually simpler than recirculation it > seems to me that it would be somewhat more difficult to implement > as it implies a two stage translation process: e.g. one stage to > determine if an ODP select group may be used; and one to perform > the translation. > > I seem to recall trying various two stage translation processes > as part some earlier unrelated work. And my recollection is that > the result of my previous efforts were not pretty. > > Both of the above more or less negate any benefits of ODP select group > action. In particular lowering flow setup cost and potentially allowing > complete offload of select groups from the datapath to hardware. However I > think that this case is not a common one as it requires both of the > following. And I think they are both not usual use cases. > > * Different buckets modifying packets in different ways > - My expectation is that it is common for buckets to be homogeneous in > regards to packet modifications. But perhaps this is na??ve in the > context of VLANs, MPLS, and similar tags that can be pushed and popped. > * Actions that rely on packet contents after > - My expectation is that it is common to use a select group to output > packets and that is the final action performed.
I am glad that you have thought about it. Your ideas seem like a good start to me. Personally, approach #2, of falling back to selecting buckets in ovs-vswitchd, seems like a clean solution to me, although if it really takes multiple stages in translation then that is undesirable, so I hope that some clean and simple approach works out. I think that we probably need a solution before we can apply the patch series, because otherwise we end up with half-working code. _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
